Education  Department 


Attractiveness  for  insects  in  showy  involucres  of  Cornus  canadensis 
After  Conway  MacMillan 


PRINCIPLES  OF 


BY 

JOSEPH  Y.  BERGEN,  A.M. 

AUTHOR  OF  "ELEMENTS  OF  BOTANY,"  "FOUNDATIONS  OF  BOTANY,' 
"  PRIMER  OF  DARWINISM,"  ETC. 


BRADLEY  M.  DAVIS,  PH.D. 

HEAD  OF  DEPARTMENT  OF  BOTANY  IN  THE  MARINE  BIOLOGICAL, 

LABORATORY,  RECENTLY  ASSISTANT  PROFESSOR  OF  PLANT 

MORPHOLOGY  IN  THE  UNIVERSITY  OF  CHICAGO 


GINN  &  COMPANY 

BOSTON  •  NEAV  YORK  •  CHICAGO  •  LONDON 


; 


COPYRIGHT,  1906,  BY 
JOSEPH  Y.  BERGEN  AND  BRADLEY  M.  DAVIS 


ALL   RIGHTS  RESERVED 
66.9 


jpresg 


G1NN   &   COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


«•£. 


PREFACE 

The  present  work  owes  its  existence  to  the  favorable  reception 
accorded  to  Bergen's  Foundations  of  Botany.  Whatever  better- 
ments have  been  suggested  by  five  years'  use  of  the  earlier  book 
in  the  hands  of  expert  teachers  will  be  found  here  incorporated. 
The  Principles  of  Botany  also  attempts  to  supply  what  many 
feel  to  be  one  of  the  most  valuable  portions  of  botany  for  edu- 
cational purposes,  namely,  a  consecutive  series  of  studies  of 
representative  spore  plants,  so  treated  as  to  outline  the  evolu- 
tionary history  of  the  plant  world.  Botanical  technology  cannot 
figure  largely  in  any  brief  general  botany.  The  authors  have  how- 
ever touched  frequently  upon  the  economic  side  of  the  subject, 
and  the  last  two  chapters  are  wholly  devoted  to  practical  topics. 

The  subject-matter  has  been  divided  into  three  parts,  treating 
respectively : 

I.   The  structure  and  physiology  of  seed  plants  (Bergen). 
II.   The  morphology,  evolution,  and  classification  of  plants,  being 
an  account  of  the  critical  morphology  of  plants  upon  which  is  based 
their  relationship  by  descent  (Davis). 

III.  Ecology  and  economic  botany  (Bergen). 

The  whole  will  furnish  material  for  a  full  year's  work,  and 
it  will  usually  be  found  necessary  to  omit  portions  and  thus 
shape  a  course  adapted  for  the  exact  conditions  under  which 
the  work  in  each  case  is  to  be  done.  It  is  not  the  intention 
of  the  authors  to  frame  an  inflexible  course,  but  rather  to  pre- 
sent in  orderly  fashion  the  material  from  which  a  thoroughly 
practical  one  can  be  planned.  Indeed,  the  authors  believe  that 
a  half-year  course  can  be  readily  arranged  by  selections  from 
the  more  general  sections  of  the  book. 

iii 


iv  PREFACE 

The  planning  of  a  course  will  be  materially  assisted  by  the 
use  of  the  authors'  Laboratory  and  Field  Manual,  which  is  so 
arranged  as  to  offer  a  choice  between  the  general  requirements 
of  a  shorter,  elementary  course  and  the  details  which  are  only 
possible  when  more  time  can  be  given  to  the  subject,  under 
excellent  conditions  of  laboratory  equipment  and  with  fairly 
mature  students.  A  glossary  of  botanical  terms  employed  in 
this  book  will  be  found  in  the  Laboratory  Manual. 

Some  instructors  will  prefer  to  devote  most  of  the  year  to  a 
study  of  seed  plants ;  others  will  choose  to  make  the  story  of 
plant  evolution  the  chief  feature  and  may  even  prefer  to  begin 
with  Part  II.  This  portion  of  the  book  is  the  outgrowth  of  ten 
years'  experience  of  the  junior  author  in  the  University  of 
Chicago,  where  he  offered  a  year's  course  in  general  morphology 
along  somewhat  similar  lines.  The  treatment  given  to  the 
thallophytes  in  Part  II  will  seem  to  some  readers  long  in  pro- 
portion to  that  allotted  to  the  other  groups  of  plants.  This 
cannot  however  be  avoided  in  any  account  which  attempts  to 
present  an  outline  of  plant  evolution  with  the  important  topics 
of  the  origin  and  evolution  of  sex  and  of  the  sporophyte.  Fur- 
thermore, it  is  very  desirable  to  describe  a  range  of  types  from 
which  selections  may  be  made  according  to  the  material  avail- 
able in  different  regions  of  the  country.  The  adaptation  of  the 
book  to  several  methods  of  approach  has  obviously  necessitated 
slight  repetitions  of  fundamental  matter  in  certain  parts. 

Whatever  the  order  of  treatment,  the  authors  would  urge  the 
importance  of  sending  the  student  to  the  plants  for  as  many  as 
may  be  of  his  facts  and  then  linking  these  together  by  read- 
ing and  class  discussion.  Undigested  laboratory  work  is  little 
better  than  none  at  all,  while  a  reading  course  without  type 
studies  and  physiological  experiments  is  a  quarter  of  a  century 
behind  the  best  practice  of  to-day.  No  matter  where  it  is  to 
end,  the  study  of  botany  should  begin  with  a  first-hand  knowl- 
edge of  plants  themselves, —  best  of  all,  with  a  knowledge  of  their 


PREFACE  V 

life  in  their  own  natural  environment.  At  the  outset  there 
may  be  far  more  botany  and  more  reasoning  power  gained  in 
finding  out  for  one's  self  the  light  relations  of  locust  or  bean 
leaves,  or  in  ascertaining  why  one  pool  is  teeming  with  Spiro- 
gyra  and  another  with  Oscillatoria,  than  in  much  reading  of 
botanical  literature. 

The  earlier  chapters  of  Part  I  are  considerably  less  difficult 
than  most  of  the  later  portions  of  the  book.  It  is  therefore 
suggested  that  care  should  be  exercised  not  to  consume  too 
much  time  in  covering  this  ground,  together  with  the  laboratory 
work  which  it  presupposes.  Classes  should  rather  be  carried 
along  somewhat  rapidly  to  such  more  difficult  topics  as  are  dis- 
cussed in  Chapters  v,  vui,  xii,  and  xv,  and  in  Part  II. 

Except  where  acknowledgment  is  made  in  the  text,  the  figures 
and  plates  are  all  new  or  from  the  Foundations  or  Elements  of 
Botany  of  the  senior  author.  Most  of  the  illustrations  of 
Part  II  are  original  and  by  Dr.  Davis.  Special  thanks  for  pho- 
tographs and  plates  either  reproduced  in  half  tone  or  redrawn 
for  zinc  etchings  are  due  to  F.  W.  Atkinson,  F.  Borgesen,  F.  E. 
Clements,  E.  M.  Freeman,  G.  L.  Goodale,  and  Conway  Mac- 
Millan.  W.  M.  Davis,  A.  E.  Frye,  and  F.  Both  have  kindly 
permitted  the  use  of  a  number  of  woodcuts  and  maps. 

Parts  of  the  manuscript  were  read  by  A.  T.  Bell,  F.  E. 
Clements,  I.  S.  Cutter,  W.  F.  Ganong,  B.  A.  Harper,  W.  M.  Hays, 
J.  C.  Jensen,  Miss  Lillian  J.  MacBae,  and  Miss  Caroline  E. 
Stringer.  Proof  was  read  by  W.  J.  Beal,  F.  E.  Clements, 
W.  N.  Clute,  I.  S.  Cutter,  H.  S.  Pepoon,  B.  M.  Stigall,  and 
Miss  Eva  0.  Sullivan.  To  all  of  these  the  authors  wish  to 
express  their  great  appreciation  for  kindly  criticisms  and  most 
helpful  suggestions.  J  Y  B 

B.M.D. 
CAMBRIDGE,  August,  1906 


CONTENTS 

PAGE 

INTRODUCTION     .     . .;....  1 

PART    I 

THE  STRUCTURE  AND  PHYSIOLOGY  OF  SEED  PLANTS 

CHAPTER 

I.  THE  SEED  AND  ITS  GERMINATION 5 

II.  THE  STORAGE  OF  FOOD  IN  THE  SEED 8 

III.  MOVEMENTS,  DEVELOPMENT,  AND  MORPHOLOGY  OF  THE  SEED- 

LING         12 

IV.  ROOTS 19 

V.  SOME    PROPERTIES   OF   CELLS  AND   THEIR   FUNCTIONS    IN  THE 

ROOT 34 

VI.  STEMS 40 

VII.  STRUCTURE  OF  THE  STEM 57 

VIII.  LIVING  PARTS  OF  THE  STEM  ;  WORK  OF  THE  STEM 71 

IX.  BUDS 80 

X.  LEAVES 88 

XL  LEAF  ARRANGEMENT  FOR  EXPOSURE  TO  SUN  AND  AIR  ;  HELIO- 

TROPIC  MOVEMENTS  OF  LEAVES  AND  SHOOTS 94 

XII.  MINUTE  STRUCTURE  OF  LEAVES  ;  FUNCTIONS  OF  LEAVES  .     .     .  102 

XIII.  THE  FLOWER  OF  THE  HIGHER  SEED  PLANTS 123 

XIV.  INFLORESCENCE 132 

XV.  ORIGIN  AND  STRUCTURE  OF  FLORAL  ORGANS  ;  POLLINATION  AND 

FERTILIZATION 138 

XVI.  THE  FRUIT   .                                                                                         .  146 


PART  II 

THE  MORPHOLOGY,  EVOLUTION,  AND  CLASSIFICATION 
OF  PLANTS 

XVII.  THE  PRINCIPLES  OF  MORPHOLOGY,  EVOLUTION,  AND  CLASSIFI- 
CATION       151 

XVIII.  THE  LOWEST  ORGANISMS  AND  THE  CELL  AS  THE  LIFE  UNIT   .     .   156 
XIX.  THE  THALLOPHYTES 172 

vii 


viii  CONTENTS 

CHAPTER  PACK 

XX.  THE  ALG^E,  THE  LOWEST  GREEN  PLANTS 173 

XXI.   SUMMARY  OF  THE  LIFE  HISTORIES  AND  EVOLUTION  OF  THE 

ALG^ 221 

XXII.  THE  FUNGI  AND  THEIR  RELATION  TO  FERMENTATION   AND 

DISEASE 227 

XXIII.  SUMMARY  OF  THE  LIFE  HISTORIES  AND  EVOLUTION  OF  THE 

FUNGI 272 

XXIV.  THE  BRYOPHYTES  AND  THE  ESTABLISHMENT  OF  ALTERNATION 

OF  GENERATIONS 275 

XXV.  THE  PTERIDOPHYTES  AND  THE  APPEARANCE  OF  HETEROSPORY  306 

XXVI.  ALTERNATION  OF  GENERATIONS 345 

XXVII.  HETEROSPORY 351 

XXVIII.  THE  SPERMATOPHYTES  AND  THE  SEED  HABIT 354 

XXIX.  THE  EVOLUTION  OF  THE  SPOROPHYTE  AND  DEGENERATION 

OF  THE  GAMETOPIIYTE  .  .     .  402 


PART  III 

ECOLOGY  AND  ECONOMIC  BOTANY 

XXX.  PARASITES  AND  CARNIVOROUS  PLANTS 407 

XXXI.   How  PLANTS  PROTECT  THEMSELVES  FROM  ANIMALS      .     .     .   413 
XXXII.  POLLINATION  OF  FLOWERS  AND  PROTECTION  OF  POLLEN  .     .  420 

XXXIII.  How  PLANTS  ARE  SCATTERED  AND  PROPAGATED 436 

XXXIV.  SOCIAL  HABITS  OF  PLANTS;  COMPETITION  AND  INVASION  .     .  447 
XXXV.  PLANT  SUCCESSIONS 454 

XXXVI.  ECOLOGICAL  GROUPS  AND  THEIR  CHARACTERISTICS      .     .     .  459 

XXXVII.  PLANT  FORMATIONS  ;  ZONATION 474 

XXXVIII.  PLANT  GEOGRAPHY 481 

XXXIX.  VARIATION,  MUTATION,  AND  ORIGIN  OF  SPECIES      ....  496 

XL.  PLANT  BREEDING 500 

XLI.  SOME  USEFUL  PLANTS  AND  PLANT  PRODUCTS      ......  514 

APPENDIX 537 

INDEX   .  .  541 


LIST  OF  PLATES 

FRONTISPIECE.  Attraction  for  pollinating  insects  in  Cornus  canadensis, 
a  shade  plant  of  cold  woods  with  inconspicuous  perianth,  but  a  large 
and  showy  white  involucre,  the  whole  head  appearing  like  a  flower. 

Faciny  page 
PLATE  I.     Sand  dunes  with  sea  rye  grass  (Elymus  arenarius).    A  sand 

binder,  deep-rooted,  with  extensively  running  rootstocks  ....  52 
PLATE  II.  Exposure  of  leaves  to  sunlight.  All  the  leaves  are  arranged 
at  such  an  angle  as  to  receive  the  maximum  illumination.  Balsam 
spruce  forest  along  a  brookside,  east  slope  of  Pikes  Peak.  Species 
represented :  Actcea  eburnea,  Chamcenerion  angustifolium,  Hera- 
cleum  lanatum,  Rubus  strigosus 94 

PLATE  III.  Cypress  swamp,  with  Spanish  moss(Tillandsia),  an  epiphytic 
seed  plant  practically  leafless,  the  work  ordinarily  done  by  leaves 
devolving  on  the  .slender  stems.  The  cypress  trees  are  furnished 
with  "knees,"  or  projections  from  the  roots,  which  are  thought 

by  some  to  absorb  air 110 

PLATE  IV.  Ilockweeds  exposed  at  low  tide 204 

PLATE  V.     A  common  tree  lichen  (Physcia  stellaris) 254 

PLATE  VI.     A  wound  parasite  (Pleurotus  ulmarius)  on  a  maple  tree      .  270 
PLATE  VII.     Tree  ferns  (Dicksonia  antarclica)  from  Tasmania     .     .     .  310 
PLATE  VIII.     A  probable  landscape  in  the  Carboniferous  Age     .     .     .  340 
PLATE  IX.     Belt  of  trees  along  a  Nebraska  river,  showing  dependence 
of  forest  on  water  supply. 

Xerophy tic  grasses  on  Nebraska  sand  hills 462 

PLATE  X.  Zonation  about  a  pond  in  southwestern  Ohio.  The  plate 
shows  the  north  end  of  a  pond  of  about  eleven  acres  in  area  par- 
tially surrounded  by  seven  zones  beginning  with  submerged  aquatics 
and  ending  with  a  forest  zone  (in  the  portion  next  the  pond,  mainly 

of  maples,  elms,  ashes,  and  willows) 478 

PLATE  XI.     Leaning  trees  at  timber  line  on  Pikes  Peak 486 

PLATE  XII.  A  coniferous  forest  interior  of  central  Colorado.  The  trees 
are  Douglas  spruce  (Psendotsuga  mucronata).  The  light  intensity 
is  so  feeble,  from  the  dense  shade,  that  the  only  seed  plants  on  the 
forest  floor  are  saprophytic  or  parasitic  orchids  and  a  few  little 
Pyrolas 494 

PLATE  XIII.  A  tropical  forest  in  the  Philippines,  showing  characteris- 
tic dense  vegetation,  the  trees  of  many  species,  mostly  palms  .  ,518 


PRINCIPLES  OF  BOTANY 


INTRODUCTION 

Botany  is  the  science  which  treats  of  plants.  It  considers  the 
structure  and  functions  of  individuals,  recognizes  their  neighbor- 
hood relations  as  citizens  of  plant  communities,  and  studies  their 
positions  as  members  of  the  plant  kingdom  more  or  less  closely 
related  by  common  descent.  The  study  of  the  individual  plant 
embraces  a  variety  of  topics,  and  the  examination  of  its  relation 
to  others  introduces  many  more  subjects. 

Morphology  is  the  science  of  form  and  structure.  Under  this 
head  are  studied  the  forms  of  plant  bodies  and  the  portions  of 
which  they  are  composed.  All  plants  except  the  very  simplest 
are  made  up  of  parts,  called  organs,  which  are  structures  devel- 
oped for  particular  kinds  of  work.  Thus  the  stems,  roots,  and 
leaves  are  organs,  and  so  are  also  the  parts  of  a  flower.  Morphol- 
ogy establishes  the  relationships  of  organs  which  seem  at  first 
glance  very  dissimilar,  as  when  leaves  take  the  form  of  bud  scales 
or  spines  or  tendrils.  Morphology  traces  the  degeneration  of 
parts  which  frequently  cease  to  perform  the  work  for  which 
they  were  originally  developed  and  become  much  simplified  in 
structure  or  almost  disappear.  Thus  the  tendrils  of  the  wood- 
bine are  shown  to  be  morphologically  branches  reduced  to  mere 
organs  of  attachment.  Although  morphology  deals  with  the 
plant  with  less  regard  to  its  character  as  a  living  being,  it 
should  never  be  entirely  separated  from  physiology,  but  should 
go  hand  in  hand  with  that  sister  subject,  equally  necessary  to 
an  understanding  of  plant  life. 


S  '".INTRODUCTION 

Plant  physiology  treats  of  the  plant  in  action,  how  it  lives, 
respires,  feeds,  grows,  and  produces  others  like  itself.  It  dis- 
cusses the  nature  of  the  material  in  which  the  life  activities  of 
the  plant  have  their  origin,  and  the  conditions  as  regards  light, 
heat,  air,  and  moisture  under  which  life  is  possible.  It  considers 
the  raw  materials  out  of  which  plant  food  is  made,  the  processes 
by  which  the  manufacture  is  carried  on,  and  the  means  by  which 
food  once  produced  is  transported  throughout  the  plant  body. 
The  mode  of  growth  of  plants  is  an  extended  and  most  impor- 
tant topic,  and  the  processes  by  which  reproduction  is  carried 
on  are  so  numerous  and  complicated  that  they  constitute  one 
of  the  most  difficult  and  interesting  departments  of  botany.  In 
order  to  go  far  into  the  details  of  the  life  activities  of  plants  one 
needs  to  know  a  good  deal  of  chemistry  and  some  physics.  But 
there  are  many  of  the  phenomena  of  plant  physiology  which 
can  be  taken  up  with  profit  in  an  elementary  way  and  investi- 
gated with  rather  simple  apparatus. 

Plant  geography  discusses  the  distribution  of  the  various 
kinds  of  plants  over  the  earth's  surface. 

Paleobotany,  usually  studied  along  with  geology,  considers 
the  history  of  plant  life  on  the  earth  from  the  appearance  of 
the  first  plants  until  the  present  time. 

Taxonomy,  or  systematic  botany,  is  concerned  with  the  classi- 
fication of  plants.  By  this  is  meant  the  arrangement  or  grouping 
of  the  kinds  of  plants  to  show  their  relationships  to  one  another. 
It  attempts  to  express  the  final  results  of  the  long  processes  of 
plant  evolution,  and  is  far  more  than  the  conventional  study  of 
flowering  plants,  which  occupy  only  the  highest  grades  in  the 
elaborate  system  of  plant  evolution  and  classification. 

Plant  ecology  treats  of  the  relations  of  the  plant  to  the  con- 
ditions under  which  it  lives,  together  with  the  origin  and  devel- 
opment of  plant  associations.  Under  this  division  of  the  science 
are  studied  the  effects  of  soil,  climate,  and  friendly  or  hostile 
animals  and  plants  on  the  external  form,  the  internal  structure, 
and  the  habits  of  plants.  The  main  lesson  to  be  learned  from 


ORDER  OF   STUDY  3 

the  study  of  ecology  is  that  the  plant  is  not  an  organism  of  fixed 
form,  structure,  and  habits,  sprung  from  a  long  line  of  precisely 
similar  ancestors  and  destined  to  leave  an  indefinite  series  of 
forms  like  itself  to  succeed  each  other  in  the  same  area.  On  the 
other  hand,  each  generation  is  a  little  more  or  less  numerous 
than  its  predecessors,  covering  more  or  less  territory  than  they 
did,  and  varying  from  them  this  way  or  that  under  the  influ- 
ence of  changing  conditions  of  life.  This  is  an  interesting  depart- 
ment of  botany,  but  it  has  to  be  studied  mainly  out  of  doors. 
Economic  botany  is  the  study  of  the  uses  of  plants  to  man. 

Many  of  the  topics  suggested  in  the  above  outline  cannot 
be  studied  in  detail  in  an  elementary  course.  It  ought,  however, 
to  be  possible  for  the  student  to  learn  a  good  deal  about  the 
simpler  facts  of  morphology  and  of  plant  physiology.  It  is 
necessary  to  study  plants  themselves,  to  take  them  to  pieces 
and  to  make  out  the  connection  of  their  parts,  to  examine  with 
the  microscope  small  portions  of  the  exterior  surface  and  thin 
slices  of  all  the  variously  built  tissues  of  which  the  plant  consists. 
Among  the  lower  plants  there  will  be  found  a  most  attractive 
study  of  cell  structure,  reproductive  processes,  and  life  histories, 
—  all  requiring  the  use  of  the  compound  microscope.  Living 
plants  must  be  watched  in  order  to  ascertain  what  kinds  of  food 
they  take,  what  kinds  of  waste  substances  they  excrete,  how  and 
where  their  growth  takes  place  and  what  circumstances  favor  it, 
how  they  move,  and  indeed  to  get  as  complete  an  idea  as  pos- 
sible of  what  has  been  called  the  behavior  of  plants. 

Since  the  most  familiar  plants  spring  from  seeds,  the  beginner 
in  botany  may  well  examine  at  the  outset  the  structure  of  a  few 
familiar  seeds,  then  sprout  them,  and  watch  the  growth  of  the 
seedlings  which  spring  from  them.  Afterwards  he  can  study  in  a 
few  examples  the  organs,  structure,  and  functions  of  seed  plants, 
trace  their  life  history,  and  so,  step  by  step,  follow  the  process 
by  which  a  new  crop  of  seeds  at  last  results  from  the  growth  and 
development  of  such  a  seed  as  that  with  which  he  began. 


4  INTRODUCTION 

After  he  has  come  to  know  in  a  general  way  about  the  struc- 
ture and  physiology  of  seed  plants,  the  student  may  become 
acquainted  with  some  typical  spore  plants.  This  will  open  up  a 
new  world,  illustrating  some  of  the  most  interesting  and  funda- 
mental principles  of  biological  science ;  for  an  understanding  of 
the  cell  theory  of  organization  and  development,  the  nature  of 
sexual  processes,  and  the  evolution  of  the  plant  kingdom  with 
its  remarkable  alternation  of  generations,  can  only  be  gained 
by  tracing  the  chief  steps  in  the  processes  through  the  various 
groups  of  algae,  fungi,  liverworts,  mosses,  and  ferns. 

For  users  of  the  book  who  wish  to  begin  in  the  autumn  with 
the  study  of  some  seed  plant  as  a  whole  the  following  scheme 
is  suggested: 

1.  Examine  a  seed  plant  in  flower,  to  get  an  idea  of  its  gross 
anatomy.    Then  study  the  development,  structure,  and  modes 
of  dissemination  of  the  fruit.    Outline   the  structure  of  seeds 
and  follow  the  germination  of  some  types.    Next  take  up  the 
structure  and  physiology  of  the  vegetative  members  of  the  plant 
body,  root,  stem,  and  leaf. 

2.  Cover  as  much  as  may  be  of  Part  II,  working  out  the 
story  of  the  evolution  of  plants. 

3.  Devote  the  remainder  of  the  year  to  study  of  floral  struc- 
tures, field  work  on  families  of  angidsperms,  ecological  topics, 
and  an  outline  of  economic  botany. 

If  desired,  the  course  in  botany  may  begin  with  the  simplest 
spore  plants,  tracing  the  evolution  of  the  plant  kingdom  through 
a  consecutive  study  of  types,  as  described  in  Part  II,  followed 
by  somewhat  detailed  work  on  the  structure  and  physiology  of 
seed  plants  (Part  I),  and  ecology  (Part  III). 


PART  I 

THE  STRUCTURE  AND  PHYSIOLOGY  OF 
SEED  PLANTS 


CHAPTEE  I 
THE   SEED  AND   ITS  GERMINATION 

The  seed.  A  seed  is  a  miniature  plant, 
or  embryo,  with  some  accessory  parts,  in  a 
resting  or  dormant  state,  and  capable  under 
suitable  conditions  of  reproducing  the  kind 
of  plant  which  bore  it. 

The  power  of  producing  seeds  is  peculiar 
to  the  higher  plants  (seed  plants,  or  sper- 
matopliytes]  and  sharply  distinguishes  them 
from  all  lower  forms  of  plant  life. 

The  embryo  may  nearly  or  quite  fill  the 
interior  of  the  seed,  as  in  Fig.  1,  or  it  may 
constitute  only  a  small  part  of  the  bulk  of 
the  latter,  as  in  Figs.  3,  4. 

2.  Form  and  position  of  the  embryo. 
The  embryo  shows  great  diversity  of  form; 
it  may  have  one,  two,  or  several  seed  leaves, 
or  cotyledons  (Figs.  1,  3,  12).  These  may 
be  straight,  as  in  the  squash  seed,  or  much 
curved  and  folded,  as  in  the  seed  of  the 
four-o'clock,  morning-glory,  or  buckwheat, 
but  they  are  almost  always  closely  packed  for  economy  of  space. 

5 


-c 


FIG.  1.  Lengthwise  sec- 
tion of  a  squash  seed 

c,  hypocotyl ;  co,  cotyle- 
don; e,  endosperm ;  h, 
hilum ;  p,  plumule; 
t,  testa.  Magnified 
about  two  and  a  half 
times 


THE  SEED  AND  ITS  GERMINATION 


The  cotyledons  are  usually  borne  on  a  little  stem,  called  the 
hypocotyl  (meaning  beneath  the  cotyledon)  (Fig.  1,  c ;  Fig.  2,  c). 
Often  a  little  seed  bud,  or  plumule  (Fig.  3),  is  easily  recogni- 
c  zable  in   the   embryo,  more   or 

less  inclosed  by  the  cotyledons, 
if  there  are  two  of  these. 

3.  The  seed  coats.    The  em- 
bryo (and  sometimes  other  seed 
contents)  is  inclosed  by  one  or 
more  seed  coats,  which  in  many 
cases  preserve  the  embryo  from 
injuries   of  various   kinds,   and 
also  serve  other  purposes.    The 
principal  seed  coat  is  called  the 
testa;  it  varies  greatly  in  thick- 
ness, hardness,  color,  and  mark- 
ings, and  also  in  other  respects, 
as  is  evident  when  one  recalls 
the  varied  appearance  of  such 
familiar   seeds  as  those  of  the 
mustard,  squash,  bean,  pea,  locust, 
apple,  poppy,  and  Brazil  nut. 

4.  Topics   for  investigation. 
The  student  should  learn  at  first 
hand  (that  is,  from  the  seeds  and 
the  young  seedlings  themselves), 

in  connection  with  the  present  chapter,  something  about  the 
following  topics : 

1.  The  anatomy  of  a  few  typical  seeds. 

2.  Some  of  the  conditions  for  germination. 

3.  Some  of  the  chemical   changes  produced  in  germinating 
seeds,  and  their  effect  upon  the  surrounding  air. 

4.  The  early  steps  in  the  development  of  seeds  into  plants. 
The  brief  outline  of  the  structure  of  the  seed  just  given  should 

be  much  enlarged  by  the  details  learned  in  the  laboratory. 


FIG.  2.   The  castor  bean  and  its 
germination 

A,  lengthwise  section  of  ripe  seed :  t, 
testa;  co,  cotyledon ;  c,  hypocotyl. 
B,  sprouting  seed  covered  with  en- 
dosperm. C,  same,  with  half  of  en- 
dosperm removed.  D,  seedling:  ?•, 
primary  root;  r',  secondary  roots; 
c,  arch  of  hypocotyl 


OXIDATION  INVOLVED  IN   GERMINATION 


Every  observing  person  who  has  grown  plants  from  the  seed 
has  learned  that  heat  and  moisture  are  necessary  to  insure  ger- 
mination, but  the  student  will  readily  discover,  too,  that  air  is 
necessary  for  anything  more  than  the 
beginning  of  germination. 

5.  Oxidation  involved  in  germina- 
tion. Germinating  seeds,  like  all  liv- 
ing things,  consume  much  oxygen,— 
the  gas  everywhere  present  in  the 
atmosphere  which  supports  the  com- 
bustion of  coal  and  other  fires  and  of 
lamps  and  gas  flames.  In  place  of  the 
oxygen  which  they  absorb,  sprouting 
seeds  return  to  the  air  carbon  dioxide, 
—  the  gas  which  is  produced  by  burn- 
ing charcoal,  and  which  is  one  of  the 
products  of  burning  most  kinds  of  fuel 
and  of  the  respiration  of  animals. 

A  thermometer  with  its  bulb  im- 
mersed in  a  jar  of  sprouting  peas 
will  mark  a  temperature  somewhat 
higher  than  that  of  the  room  in  which  they  stand.  The  eleva- 
tion of  temperature  is  at  least  partly  due  to  the  union  of 
oxygen  with  combustible  materials  in  the  peas.  Such  a  combi- 
nation is  known  as  oxidation.  This  kind  of  chemical  change 
is  universal  in  plants  and  animals  while  they  are  in  an  active 
condition,  and  the  energy  which  they  manifest  in  their  growth 
and  movements  is  as  directly  the  result  of  the  oxidation  going 
on  inside  them  as  the  energy  of  a  steam  engine  is  the  result 
of  the  burning  of  coal  or  other  fuel  under  its  boiler.  In  the 
sprouting  seed,  much  of  the  energy  produced  by  the  action  of 
oxygen  upon  oxidizable  portions  of  its  contents  is  expended  in 
producing  growth,  but  some  of  this  energy  is  wasted  by  being 
transformed  into  heat  which  escapes  into  the  surrounding  soil. 
It  is  this  escaping  heat  which  is  detected  by  the  thermometer. 


FIG.  3.    Lengthwise    section 
of  grain  of  corn 

y,  yellow,  proteid  part  of  endo- 
sperm ;  w,  white,  starchy  part 
of  endosperm;  p,  plumule; 
s,  the  shield  (cotyledon),  in 
contact  with  the  endosperm 
for  absorption  of  food  from 
it;  r,  the  primary  root. 
Magnified  about  three  times. 
—  After  Sachs 


CHAPTEE  II 


A  B 

FIG.  4.    Seeds  with   endo- 
sperm, longitudinal  sections 

A,  asparagus  (magnified) ;  Z?, 
poppy  (magnified).  —After 
Decaisne 


THE  STORAGE  OF  FOOD  IN  THE  SEED 

6.  Importance  of  stored  food  for  growth  of  the  seedling.    A 
very  large  part  of  the  food  of  man  and  of  many  of  the  higher 

animals  consists  of  seeds  of  various 
kinds,  particularly  of  the  grains.  Every 
kind  of  seed  contains  some  stored  food 
material,  though  the  amount  in  the 
poppy  seed  is  but  an  insignificant  frac- 
tion of  that  in  a 
horse-chestnut. 
Very  often,  as 
has  already  been 
learned,  the  food 

is  stored  directly  in  the  embryo,  espe- 
cially in  the  cotyledons.  Frequently,  how- 
ever, most  of  it  is  deposited  in  the  en- 
dosperm, which  surrounds  or  lies  along- 
side of  the  cotyledons  (Figs.  2,  3,  4).  In 
either  case  the  slow  germination  and  sub- 
sequent growth  of  seeds  from  which  part 
or  all  of  the  food  material  has  been 
removed  shows  that  its  presence  is  most 
important  in  forcing  along  the  growth 
of  the  seedling  (Fig.  5). 

7.  Usefulness  of  rapid  growth  of  seed- 
lings.   The  very  existence  of  the  young 

plant  may  depend  upon  its  being  able  to  FIG. 5.  Germinating 
make  a  rapid   start  in  life.    Most  areas 
of  fertile  land  contain    far   more    seeds 

8 


peas,  growing  in  water, 
one  deprived  of  its 
cotyledons 


STARCH 


9 


than  can  mature  plants  under  the  conditions  of  competition 
with  one  another  which  they  must  encounter,  and  so  plants 
which  shoot  up  rapidly  at  first  possess  a  decided  advantage. 
There  is  also  a  much  better  chance  for  seedlings  growing  in 
woodlands  if  they  can  attain  considerable  size  before  they  are 
too  much  shaded  by  the  foliage  of  the  trees  above  them.  This, 
of  course,  does  not 
apply  to  evergreen 
woods. 

8.  Kinds  of  food 
stored  in  seeds. 
The  three  princi- 
pal kinds  of  plant 
food,  or  reserve 
material  stored  in 
seeds,  are  starch, 
oil,  and  albumi- 
nous substances, 
orproteids.1  A  sin- 


gle  seed  may  con- 
tain all    three    of 


FIG.  6.   Section  through  exterior  part  of  a 
grain  of  wheat 


these  in   consider-    c'  cuticle>  or  outer  layer  of  bran;    ep,  epidermis;    ra, 
middle  layer;  i,  i^,  layers  of  hull  next  to  seed  coats; 

able    proportions,       s,  «i,  seed  coats;  p,  layer  containing  proteid  grains; 
Or  it   may  Contain       st>  cel1*  °f  the  endosperm  filled  with  starch.    Greatly 
J  magnified.  —  After  Ischirch 

proteids  together 

with  either  starch  or  oil.  Some  proteids  are  always  present, 
since  the  power  of  the  seed,  to  live  and  grow  depends  upon 
these  compounds. 

9.  Starch.  Every  one  is  familiar  with  the  appearance  of  starch 
in  its  commercial  form.  As  found  in  seeds  it  occurs  in  micro- 
scopic compartments  known  as  cells  (Fig.  6).  Each  cell  contains 
many  small  starch  grains,  usually  of  a  nearly  round  or  an  ovoid 

1  As  in  general  throughout  the  book,  the  statements  of  the  text  pre- 
suppose a  suitable  amount  of  laboratory  work ;  for  example,  that  of  the 
manual  of  the  authors. 


10 


THE  STORAGE  OF  FOOD  IN  THE  SEED 


FIG.  7.    Canna  starch 
Magnified  300  diameters 


shape.  The  shape  and  markings  of  a  starch  grain,  whether 
found  in  the  seed  or  in  some  other  part  of  the  plant,  are  often 
sufficiently  definite  to  serve  to  identify  the  kind  of  plant  from 
which  they  came.  Frequently  the 
markings  are  very  regular  and  beauti- 
ful, as  in  canna  starch  (Fig.  7).  They 
are  due  to  the  successive  layers  de- 
posited as  the  starch  grain  is  formed. 
During  the  growth  of  the  seedling, 
seeds  containing  starch  rapidly  lose 
it,  and  microscopical  examination  of 
a  sprouting  grain  of  corn  or  of  the 
cotyledons  of  a  bean  plant  several 
weeks  old  shows  the  cells  compar- 
atively emptied  of  starch  and  those 
grains  which  remain  much  eaten  away,  as  described  below. 

10.  Action  of  ferments.  A  substance  which  can  produce  or 
excite  any  one  of  the  chemical  changes  known  as  fermentation 
is  called  a  ferment.  The  most  familiar  kinds  of  fermentation 
are  the  alcoholic,  by  which  alcohol  is  produced,  and  the  acid,  by 
which  solutions  of  alcohol  (such  as  fermented  cider)  are  turned 
into  vinegar,  and  by  which  the  sugar  of  milk  is  changed  into 
lactic  acid  when  sweet  milk  turns  sour. 

All  these  fermentations  and  many  others  are  caused  by  the 
development  within  the  fermenting  substances  of  minute  living 
organisms,  either  yeasts  or  bacteria,  described  in  Chapter  xxn, 
which  are  consequently  known  as  organized  ferments. 

There  is  a  class  of  substances  which,  without  the  presence  of 
yeasts  or  bacteria,  can  produce  active  fermentation.  From  the 
absence  of  the  organisms  above-mentioned,  these  are  called 
unorganized  ferments,  and  they  are  also  known  as  enzymes. 
One  of  these,  diastase,  plays  a  most  important  part  in  seeds 
during  germination,  transforming  starch  into  sugar.  Diastase  is 
found  in  considerable  quantities  in  malt,  which  is  barley  sprouted 
and  then  quickly  killed  by  moderate  heating.  Naturally,  as  a 


OTHER  CONSTITUENTS  OF  SEEDS  11 

result  of  the  action  of  its  diastase,  malt  tastes  much  sweeter 
than  barley.  The  capacity  of  this  enzyme  to  change  starch  to 
sugar  is  extraordinary,  any  quantity  of  diastase  sufficing  to  trans- 
form ten  thousand  times  its  weight  of  starch. 

11.  Oil.    Oil  occurs  in  many  seeds  —  as,  for  example,  flax,  cot- 
ton seed,  and  corn — in  sufficient  quantity  to  make  it  worth  while 
to  extract  it  by  pressure.  •  It  may  be  seen  under  the  microscope  in 
extremely  minute  droplets,  inclosed  in  the  cells  of  certain  regions 
of  the  seed. 

12.  Proteids.    Sometimes,  as  in  Fig.  6,  at  p,  the  proteid  con- 
stituents of  the  seed  occur  in  more  or  less  regular  grains,  but 
often  they  have  no  well-defined  form  and  size.    They  have  a 
chemical  composition  very  similar  to  that  of  white  of  egg  or 
the  curd  of  milk,  and  when  scorched  produce  the  familiar  smell 
of  burnt  hair  or  feathers,  which  serves  as  a  rough  test  for  their 
presence. 

13.  Other  constituents  of  seeds.    Besides  starch,  oil,  and  pro- 
teids,  other  substances  occur  in  different  seeds.    Seme  of  these 
are  of  use  in  feeding  the  seedling,  others  are  of  value  in  protect- 
ing the  seed  itself  from  being  eaten  by  animals  or  in  rendering 
it  less  liable  to  decay.    In  such  seeds  as  that  of  the  nutmeg, 
the  essential  oil  which  gives  it  its  characteristic  flavor  probably 
makes  it  unpalatable  to  animals  and  at  the  same  time  preserves 
it  from  decay. 

Date  seeds  are  so  hard  and  tough  that  they  cannot  be  eaten 
and  do  not  readily  decay.  Lemon,  orange,  horse-chestnut,  and 
buckeye  seeds  are  too  bitter  to  be  eaten,  and  the  seeds  of  the 
apple,  cherry,  peach,  and  plum  are  somewhat  bitter. 

The  seeds  of  larkspur  (Datura}}  croton,  the  castor-oil  plant, 
mix  vomica,  and  many  other  kinds  of  plants,  contain  active 

poisons. 

1  Commonly  called  Jimson  weed. 


CHAPTER  III 


MOVEMENTS,    DEVELOPMENT,    AND    MORPHOLOGY    OF  THE 

SEEDLING 

14.  How  the  seedling  breaks  ground.  As  the  student  has 
already  learned  by  his  own  observations,  the  seedling  does  not 
always  push  its  way  straight  out  of  the  ground.  Corn,  like  all 
the  other  grains  and  grasses,  sends  a  tightly  rolled,  pointed  leaf 
vertically  upward  into  the  air ;  but  seedlings 
in  general  are  not  found  to  do 
anything  of  the  sort.  The  squash 
seedling  is  a  good  one  in  which 


A  B   •       c  D          E 

FIG.  8.   Successive  stages  in  the  life  history  of  the  squash  seedling 

GG,  the  surface  of  the  ground ;  r,  primary  root ;  r',  secondary  root ;  c,  hypocotyl ; 
a,  arch  of  hypocotyl ;  co,  cotyledons 

to  study  what  may  be  called  the  arched  type  of  germination. 
If  the  seed  when  planted  is  laid  horizontally  on  one  of  its  broad 
surfaces,  it  usually  goes  through  some  such  changes  of  position 
as  are  shown  in  Fig.  8. 

The  seed  is  gradually  tilted  until,  at  the  time  of  their  emer- 
gence from  the  ground  (at  C),  the  cotyledons  are  almost  ver- 
tical. The  only  part  above  the  ground  line  GG,  at  this  period, 

12 


MOVEMENTS  OF   THE   COTYLEDONS  13 

is  the  arched  hypocotyl.  Once  out  of  ground,  the  cotyledons 
soon  rise,  until  (at  E)  they  are  again  vertical,  but  with  the 
other  end  up  from  that  which  stood  highest  in  C.  Then  the 
two  cotyledons  separate  until  they  once  more  lie  horizontally, 
pointing  away  from  each  other. 

Whether  the  first  part  of  the  seedling  to  emerge  from  the 
ground  is  a  pointed,  rolled-up  leaf,  as  in  Indian  corn,  or  the  hypo- 
cotyl arch,  as  in  Figs.  2  and  8,  the  result  is  to  force  the  earth 
aside  without  injury  to  the  plumule  or  the  cotyledons. 

15.  What  pushes  the  cotyledons  up?   A  very  little  study  of 
any  set  of  squash  seedlings,  or  even  of  Fig.  8,  is  sufficient  to 
show  that  the  portion  of  the  plant  where  roots  and  hypocotyl 
are  joined  neither  rises  nor  sinks,  but  that  the  plant  grows  both 
ways  from  this  part  (a  little  above  rr  in  Fig.  8,  A  and  B).    It  is 
evident  that  as  soon  as  the  hypocotyl  begins  to  lengthen  much 
it  must  do  one  of  two  things:  either  push  the  cotyledons  out 
into  the  air  or  else  force  the  root  down  into  the  ground  as  one 
might  push  a  stake  down.     The  plantlet,  in  passing  from  the 
stage  shown  at  A  to  that  of  B  and  of  C,  develops  many  lateral 
roots,  thus  making  it  harder  and  harder  for  the  root  to  be  thrust 
bodily  downward. 

16.  Getting  rid  of  the  seed  coats.    In  seeds  with  thin  coats 
the  latter  usually  burst  open  irregularly  and  allow  the  opening 
cotyledons  to  escape.    But  in  seeds  with  as  thick  a  testa  as  that 
of  the  squash,  and  still  more  in  the  case  of  nuts,  the  cotyledons 
find  their  way  out  through  a  slit,  or  opening,  which  appears  in 
a  definite  part  of  the  seed.    If  for  any  reason  the  seed  coat  does 
not  open,  the  embryo  cannot  grow.    In  many  cases  the  moisture 
and  freezing  and  thawing  of  a  winter  in  the  earth  are  almost 
essential  to  germination,  and  some  seeds  grow  more  promptly  if 
they  have  been  scorched  by  fire,  or  if  they  are  cracked  open 
before  planting. 

17.  Discrimination  between  root  and  hypocotyl.    It  is  not 
always  easy  to  decide  by  their  appearance  and  behavior  what 
part  of  the  seedling  is  root  and  what  part  is  hypocotyl.    In  a 


14  MORPHOLOGY  OF  THE  SEEDLING 

seedling  visibly  beginning  to  germinate,  the  sprout,  as  it  is  com- 
monly called,  which  projects  from  the  seed  might  be  either  root 
or  hypocotyl,  or  might  consist  of  both  together,  so  far  as  its 
appearance  is  concerned.  A  microscopic  study  of  the  cross  sec- 
tion of  a  root,  compared  with  one  of  the  hypocotyl,  would  show 
decided  differences  of  structure  between  the  two.  Their  mode 
of  growth  is  also  different,  as  the  pupil  may  infer  from  his  own 
observations. 

18.  Final  position  of  the  cotyledons.    As  soon  as  the  young 
plants  of  squash,  bean,  and  pea  have  reached  a  height  of  three 
or  four  inches  above  the  ground,  it  is  easy  to  recognize  important 
differences  in  the  way  in  which  they  set  out  in  life. 

The  cotyledons  of  the  squash  increase  greatly  in  surface, 
acquire  a  green  color  and  a  generally  leaf-like  appearance,  and, 
in  fact,  do  the  work  of  ordinary  leaves.  In  such  a  case  as  this 
the  appropriateness  of  the  name  seed  leaf  is  evident  enough,  — 
one  recognizes  at  sight  the  fact  that  the  cotyledons  are  actually 
the  plant's  first  leaves.  In  the  bean  the  leaf-like  nature  of  the 
cotyledons  is  not  so  clear.  They  rise  out  of  the  ground  like  the 
squash  cotyledons,  but  then  gradually  shrivel  away,  though  they 
may  first  turn  green  and  somewhat  leaf-like  for  a  time. 

In  the  pea,  as  in  the  acorn,  the  horse-chestnut,  and  many 
other  seeds,  we  have  quite  another  plan,  —  the  underground  type 
of  germination.  Here  the  thick  cotyledons  no  longer  rise  above 
ground  at  all,  because  they  are  so  gorged  with  food  that  they 
could  never  become  leaves ;  but  the  young  stem  pushes  rapidly 
up  from  the  surface  of  the  soil. 

19.  Development  of  the  plumule.    The  development  of  the 
plumule  seems  to  depend  somewhat  on  that  of  the  cotyledons. 
The  squash  seed  has  cotyledons  which  are  not  too  thick  to 
become  useful  leaves,  and  so  the  plant  is  in  no  special  haste  to 
get  ready  any  other  leaves.    The  plumule,  therefore,  cannot  be 
found  with  the  magnifying  glass  in  the  unsprouted  seed,  and  is 
almost  microscopic  in  size  at   the  time  when    the  hypocotyl 
begins  to  show  outside  of  the  seed  coats. 


ROOT,   STEM,   AND  LEAF  15 

In  the  bean,  pea,  and  corn,  on  the  other  hand,  since  the  cotyle- 
dons cannot  serve  as  foliage  leaves,  the  later  leaves  must  be 
pushed  forward  rapidly.  In  the  bean  the  first  pair  are  already 
well  formed  in  the  seed.  In  the  pea  they  cannot  be  clearly 
made  out,  since  the  young  plant  forms  several  scales  on  its  stem 
before  it  produces  any  full-sized  leaves,  and  the  embryo  contains 
only  hypocotyl,  cotyledons,  and  a  sort  of  knobbed  plumule,  well 
developed  in  point  of  size,  representing  the  lower,  scaly  part  of 
the  stem. 

20.  Root,  stem,  and  leaf.    By  the  time  the  seedling  is  well 
out  of  the  ground  the  plant  body,  in  most  cases,  possesses  the 
three  kinds  of  vegetative  organs,  or  parts  essential  to  growth,  of 
ordinary  seed  plants  ;  that  is,  the  root,  stem,  and  leaf,  or,  as  they 
are  sometimes  classified,  root  and  shoot.    All  of  these  organs 
may  multiply  and  increase  in  size  as  the  plant  grows  older,  and 
their  mature  structure  will  be  studied  in  later  chapters ;  but 
some  facts  concerning  them  can  best  be  learned  by  watching 
their  growth  from  the  outset. 

21.  Elongation  of  the  root.    We  know  that  the  roots  of  seed- 
lings grow  pretty  rapidly  from  the  fact  that  each  day  finds  them 
reaching  visibly  farther  down  into  the  water  or  other  medium 
in  which  they  are  planted.    A  sprouted  Windsor  bean  in  a  ver- 
tical thistle  tube  will  send  its  root  downward  fast  enough  so 
that  ten  minutes'  watching  through  the  microscope  will  suffice 
to  show  growth. 

22.  Root  hairs.    Very  young  seedlings  of  the  grains,  or  of 
mustard  or  red  clover,  afford  convenient  material  for  studying 
root  hairs.    These  are  most  abundantly  developed    when   the 
seed  is  sprouted  in  air  that  is  not  very  moist.    Only  a  certain 
zone  of  the  young  root  is  covered  with  live  hairs ;  the  younger 
portions  have  not  developed  them  and  the  older  portions  show 
only  dead  ones.    Examination  with  a  good  lens  or  a  low  power 
of  the  microscope  shows  the  gradual  lengthening  of  the  hairs, 
from  very  young  ones  near   the  root  tip  to  full-grown  ones 
farther  up. 


16 


MORPHOLOGY  OF  THE  SEEDLING 


The  root  hairs  in  plants  growing  under  ordinary  conditions 
are  surrounded  by  the  moist  soil  and  wrap  themselves  around 
microscopical  particles  of  earth  (Fig.  9).  Thus  they  are  able  rap- 
idly to  absorb  through  their  thin  walls  the  soil  water,  with  what- 
ever mineral  substances  it  has  dissolved  in  it. 

23.  The  young  stem.  The  hypocotyl,  or  portion  of  the  stem 
which  lies  below  the  cotyledons,  is  the  earliest  formed  portion 


FIG.  9 
FIG.  9.    Cross  section  of  a  root 


FIG.  10 


A  good  deal  magnified,  showing  root  hairs  attached  to  particles  of  soil,  and  some- 
times enwrapping  these  particles.  —  After  Frank  and  Tschirch 

FIG.  10.   A  turnip  seedling,  with  the  cotyledons  developed  into 
temporary  leaves 

h,  root  hairs  from  the  primary  root ;  b,  bare  portion  of  the  root,  on  which  no  hairs 
have  as  yet  been  produced 

of  the  stem.  Sometimes  this  grows  but  little ;  often,  however, 
the  hypocotyl  lengthens  enough  to  raise  the  cotyledons  well 
above  ground,  as  in  Fig.  10. 

The  later  portions  of  the  stem  are  considered  to  be  divided 
into  successive  sections  called  nodes  (places  at  which  a  leaf, 
or  a  scale  which  represents  a  leaf,  appears)  and  internodes 
(portions  between  the  leaves). 


THE  FIRST  LEAVES 


17 


The  stem  increases  in  length  by  the  simultaneous  elongation 
of  several  internodes,  as  shown  by  Fig.  11.  It  will  be  noticed 
that  in  the  plant  figured  the  greatest  increase  in 
length  js  neither  in  the  oldest  nor  the  youngest 
internodes  which  are  growing  at  all,  but  in  an  inter- 
mediate region. 

Every  portion  of  the  entire  shoot,  shown  in  the 
figure,  has  elongated  except  the  interval  21-22. 

Counting  from  the  top  the  lengthening  of  several 
of  the  segments  is  as  follows : 


INTERW 

1 
o 

3 
4 

5 
6 
7 
8 
9 
10 
11 

24. 

are,  as 
which 

PER  CENT  INCREASE 
IN  LENGTH 
100                     <4 

.      .      .      120 

140                  V 

Xfc 

140 

.      .     .     160 

140 

120 

.     .           .     .     .     .     110 

.     .     .     110 

100 

80 

The  first  leaves.    The  cotyledons     v^ 
already  explained,  the  first  leaves       >J 
the  seedling  possesses.    Even  if           A 

a  plumule  is  found  well  developed  in   FIG.  11.  Growth  in  a  hori- 
zontal   shoot  of    hedge 
the  seed,  it  was  formed  after  the  coty-      bindweedi 


ledons.    In  those  plants  which  have  so 

much  food  stored  in  the  cotyledons  as  to 

render  them  unfit  ever  to  become  useful 

foliage  leaves,  as   in  the   pea,  there  is 

little  or  nothing  in  the  color,  shape,  or  general  appearance  of 

the  cotyledon  to  make  one  think  it  really  a  leaf,  and  it  is  only 

by  studying  many  cases  that  the  botanist  is  enabled  to  class  all 


the  shoot  divided  by 
ink  marks  into  22  equal 
segments  ;  I>,  the  same, 
twenty-four  hours  later. — 
After  Bonnier  and  Sablon 


1  Convolvulus  sepium. 


18  MORPHOLOGY  OF  THE  SEEDLING 

cotyledons  as  leaves  in  their  nature,  even  if  they  are  quite 
unable  to  do  the  ordinary  work  of  leaves.  In  seeds  which  have 
endosperm,  or  food  stored  outside  of  the  embryo,  the  cotyledons 
usually  become  green  and  leaf-like,  as  they  do,  for  example,  in 
the  four-o'clock,  the  morning-glory,  and  the  buckwheat ;  but  in 
the  seeds  of  the  true  grains,  which  contain  endosperm,  as  in  the 
familiar  instance  of  Indian  corn,  a  large  portion  of  the  single 
cotyledon  remains  throughout  as  a  thickish  mass  buried  in  the 
seed.  In  a  few  cases,  as  in  the  pea,  there  are  scales  instead  of 
true  leaves  formed  on  the  first  nodes  above  the  cotyledons,  and 
co  it  is  only  at  about  the  third  node  above  that 

leaves  of  the  ordinary  kind  appear.  In  the  bean 
and  some  other  plants  which  in  general  bear  one 
leaf  at  a  node  along  the  stem,  there  is  a  pair 
produced  at  the  first  node  above  the  cotyledons, 
and  the  leaves  of  this  pair  differ  in  shape  from 
those  which  arise  from  the  succeeding  portions 
of  the  stem. 

25.  Classification  of  plants  by  the  number  of 
V  their  cotyledons.    In  the  pine  family  the  germi- 

FIG.  12.  Germi-  natiug  seed  often  displays  more  than  two  coty- 
ledons, as  shown  in  Fig.  12  ;  in  the  majority  of 
common  seed  plants  the  seed  contains  two  coty- 
ledons, while  in  the  lilies,  the  rushes,  the  sedges,  the  grasses, 
and  some  other  plants  there  is  but  one  cotyledon.  Upon  these 
facts  is  based  the  division  of  most  seed  plants  into  two  great 
groups :  the  dicotyledonous  plants,  which  have  two  seed  leaves, 
and  the  monocotyledonous  plants,  which  have  one  seed  leaf. 
Other  important  differences  nearly  always  accompany  the  differ- 
ence in  number  of  cotyledons,  as  will  be  seen  later. 


CHAPTER   IV 


ROOTS 

26.  Origin  of  roots.    The  primary  root  originates  from  the 
lower  end  of  the  hypocotyl,  as  the  student  learned  from  his  own 
observations  on  sprout- 
ing seeds.    The  branches 

of  the  primary  root  are 
called    secondary    roots, 
and  the  branches   of 
these  are  known  as 
tertiary  roots.     Those 
roots  which  occur  on  the 
stem  or  in  other  unusual 
places  are  known  as  ad- 
ventitious roots.    The 
roots  which  form   so 
readily    on    cuttings   of 
willow,  southernwood, 
Tropseolum,  French  marigold, 
cultivated  "  geranium  "  (Pelargo- 
nium), Tradescantia,  and  many 
other    plants,  when    placed    in 
damp  earth  or  water,  are  adven- 
titious. 

27.  Aerial  roots.    While   the 
roots    of    most    familiar    plants 

grow  in  the  earth,  there  are  others  which  are 
formed  in  the  air,  called  aerial  roots.  They  serve  various  pur- 
poses :  in  some  tropical  air  plants  (Fig.  1 3)  they  fasten  the  plant 
to  the  tree  on  which  it  establishes  itself,  as  well  as  take  in 

19 


FIG.  13. 

Aerial  roots  of 

an  orchid 


20 


ROOTS 


water  which  drips  from  branches  and  trunks  above  them,  so 
that  these  plants  require  no  soil  and  grow  suspended  in  mid  air 
from  trees  which  serve  them  merely  as  supports ;  many  such 
air  plants  are  grown  in  greenhouses.  In  such  plants  as  the  ivy 
(Fig.  14)  the  aerial  roots,  which  are  also  adventitious,  hold  the 
plant  to  the  wall  or  other  surface  up  which  it  climbs. 

In  the  Indian  com  (Fig.  15)  roots  are  sent  out  from  nodes  at 
some  distance  above  the  ground  and  descend  until  they  enter 


FIG.  14.   Aerial,  adventitious  roots  of  the  ivy 

the  ground.  They  serve  to  anchor  the  cornstalk  so  that  it  may 
resist  the  wind,  and  to  supply  additional  water  to  the  plant.  They 
often  produce  no  rootlets  until  they  reach  the  ground. 

28.  Water  roots.  Many  plants,  such  as  the  willow,  readily 
adapt  their  roots  to  live  either  in  earth  or  in  water,  and  some, 
like  the  little  floating  duckweed,  regularly  produce  roots  which 
are  adapted  to  live  in  water  only.  These  water  roots  often  show 
large  and  distinct  sheaths  on  the  ends  of  the  roots,  as,  for 
instance,  in  the  so-called  water  hyacinth  (Eichhorma). 


AERIAL  ROOTS 


21 


FIG.  15.   Lower  part  of  stem  and  roots  of  Indian  corn,  showing  aerial  roots 

("brace  roots") 

v-  c,  internodes  of  the  stem ;  6,  d,  e,f,  nodes  of  various  age  hearing  roots.  Most 
of  these  started  as  aerial  roots,  hut  all  except  those  from  6  have  now  reached  the 
earth 


22  KOOTS 

29.  Parasitic  roots.  The  dodder,  the  mistletoe,  and  a  good 
many  other  seed  plants  are  called  parasites,  since  they  live,  at 
least  in  part,  upon  food  which  they  steal  from  other  plants 


ABC 

FIG.  16.   Dodder,  a  parasitic  seed  plant 

A,  magnified  section  of  stem  penetrated  by  roots  of  dodder;  B,  dodder  upon 
a  golden-rod  stem ;  (7,  seedling  dodder  plants  growing  in  earth ;  h,  stem  of 
host;  I,  scale-like  leaves;  r,  sucking  roots,  or  haustoria;  s,  seedlings. — 
A  and  C  after  Strasburger 

called  their  hosts.  Parasites  develop  peculiar  roots,  which  pene- 
trate the  tissues  of  the  host  and  form  most  intimate  connections 
with  the  interior  portions  of  the  stem  or  root  of  the  latter. 


FORMS  OF  ROOTS 


23 


In  the  dodder,  as  is  shown  in  Fig.  1 6,  the  seedling  parasite  is 
admirably  adapted  to  the  conditions  under  which  it  is  to  live. 
Hooted  at  first  in  the  ground,  it  develops  a  slender,  leafless  stem, 
which,  leaning  this  way  and  that,  no  sooner  comes  into  perma- 
nent contact  with  a  congenial  host  than  it  produces  sucking 
roots  at  many  points,  gives  up  further  growth  in  its  soil  roots, 
and  lengthens  rapidly  on  the  strength  of  the  supplies  of  ready- 
made  sap  which  it  obtains  from  the  host. 

30.  Forms  of  roots.  The  primary  root  is  that  which  proceeds 
like  a  downward  prolongation  directly  from  the  lower  end  of  the 


FIG.  17  FIG.  18  FIG.  19 

A  tap  root  Fibrous  roots      Fleshy  and  clustered  roots 

hypocotyl.  In  many  cases  the  mature  root  system  of  the  plant 
contains  one  main  root  much  larger  than  any  of  its  branches. 
This  is  called  a  tap  root  (Fig.  17). 

Such  a  root,  if  much  thickened,  may  assume  some  such  form 
as  that  of  the  carrot,  parsnip,  beet,  turnip,  or  radish,  and  is  called 
a  fleshy  root.  Some  plants  produce  a  cluster  of  roots  from  the 
lower  end  of  the  hypocotyl.  Such  roots  often  become  thickened, 
as  in  the  sweet  potato  and  the  dahlia  (Fig.  19). 

Eoots  of  grasses,  etc.,  are  thread-like,  and  known  as  fibrous 
roots  (Fig.  18). 


24 


ROOTS 


31.  General  structure  of  roots.  The  general  structure  of  the 
very  young  root  can  be  partially  made  out  by  examining  the 
entire  root  with  a  moderate  magnifying  power.  Often  the  whole 
is  sufficiently  translucent  to  allow  the  interior  as  well  as  the 

exterior  portion  to  be 
studied  while  the  root 
is  still  alive  and  grow- 
ing. 

The  main  bulk  of 
the  root  is  composed  of 
a  central  cylinder  and 
the  cortical  portion 
which  surrounds  it. 
The  outermost  part  of 
the  cortex  is  a  layer  of 
cells  forming  a  thin 
skin  known  as  the  epi- 
dermis. The  tip  of  the 
root  is  covered  by  a 
mass  of  loosely  attached 
cells  forming  the  pro- 
tective root  cap. 

On  examining  Figs. 
FIG.  20.    Lengthwise    section    (somewhat  dia-  .. 

grammatic)  through  root  tip  of  Indian  corn.    20   and   21»  the   C7lm- 
x  about  130  ders  of  which  the  root 

W,  root  cap ;  i,  younger  part  of  cap ;  z,  dead  cells  is    made    up  are   easily 

separating  from  cap;  *,  growing  point;  o,  epi-  j^ncrni^TipH     nnr|    thp 

dermis ;  p',  intermediate  layer  between  epidermis  °  BQ>  a 

and  central  cylinder ;  p,  central  cylinder,  in  which  main  Constituent  parts 
the  fib ro- vascular  bundles  arise .  —  A  f  ter  Wiesner      „ 

of  each  can  be  made 

out  without  much  trouble.  The  epidermal  cells  are  seen  to  be 
somewhat  brick- shaped,  many  of  them  provided  with  extensions 
into  root  hairs.  Inside  the  epidermis  lie  several  layers  of  rather 
globular,  thin-walled  cells,  and  inside  these  a  boundary  layer 
between  the  cortical  or  bark  portion  of  the  root  and  the  central 
cylinder.  This  latter  region  is  especially  marked  by  the  presence 


STORAGE  OF  RESERVE  MATERIAL  IN  ROOTS        25 


of  certain  groups  of  cells,  shown  at  w,  d,  and  b  (Fig.  21),  the  two 
former  serving  as  channels  for  air  and  water,  the  latter  (and  w 
also)  giving  toughness  to  the  root. 

Eoots  of  shrubs  and  trees  more  than  a  year  old  will  be  found  to 
have  increased  in  thickness  by  the  process  described  in  Chapter 
vii,  and  a  section  may  look  unlike  that  shown  in  Fig.  21. 

32.  Storage  of  re- 
serve material  in 
roots.  Many  roots 
contain  large  quanti- 
ties of  stored  plant 
food,  usually  in  the 
shape  of  starch,  sugar, 
proteids,  or  all  three 
together.  Parsnips, 
carrots,  turnips,  and 
sweet  potatoes  are 
familiar  examples  of 
storage  roots. 

Beet  roots  contain 
so  much  sugar  that 
a  large  part  of  the 
sugar  supply  of 
Europe,  and  an  in- 
creasing portion  of 
our  own  supply,  is 
obtained  from  them.  Oftentimes  the  bulk  of  a  fleshy  root  is 
exceedingly  large  as  compared  with  that  of  the  parts  of  the 
plant  above  ground. 

Not  infrequently  roots  have  a  bitter  or  nauseous  taste,  as  in 
the  case  of  the  chicory,  the  dandelion,  and  the  rhubarb ;  and  a 
good  many,  like  the  monkshood,  the  yellow  jasmine,  and  the 
pinkroot,  are  poisonous.  Evidently  the  plant  may  be  benefited 
by  the  disgusting  taste  or  poisonous  nature  of  its  roots,  which 
renders  them  uneatable. 


FIG.  21.  Much  magnified  cross  section  of  a  young 
dicotyledonous  root 

h,  root  hairs  with  adhering  bits  of  sand;  e,  epi- 
dermis; 5,  thin-walled,  nearly  globular  cells  of 
bark;  b,  hard  bast;  c,  cambium;  10,  wood  cells; 
d,  ducts 


26 


ROOTS 


33.  Use  of  the  food  stored  in  fleshy  roots.  The  parsnip, 
beet,  carrot,  and  turnip  are  biennial  plants ;  that  is,  they  do 
not  produce  seed  until  the  second  summer  or  fall  after  they  are 
planted. 

The  first  season's  work  consists  mainly  in  producing  the  food 
which  is  stored  in  the  roots.  To  such  storage  is  due  their  char- 
acteristic fleshy  appearance.  If  the  root  is  planted  in  the  fol- 
lowing spring,  it  feeds  the  rapidly  growing  stem  which  proceeds 
from  the  bud  at  its  summit,  and  an  abun- 
dant crop  of  flowers  and  seed  soon  follows; 
while  the  root,  if  examined  in  late  summer, 
s=^;*f:,-'^fom  wiH  ^e  found  to  be  withered,  with  its  store 
v  ^^S^MT\  °^  reserve  matei>i-al  quite  exhausted. 
\/\  Cff  \  The  roots  of  the  rhubarb  (Fig.  22),  the 

sweet  potato,  and  of  a  multitude  of  other 
perennials,  or  plants  which  live  for  many 
years,  contain  much  stored  plant  food. 
Many  such  plants  die  to  the  ground  at  the 
beginning  of  winter,  and  in  spring  make  a 
rapid  growth  from  the  materials  laid  up  in 
the  roots. 

34.  Extent  of  the  root  system.  The  total 
length  of  the  roots  of  ordinary  plants  is 
much  greater  than  is  usually  supposed. 
They  are  so  closely  packed  in  the  earth  that 
only  a  few  of  the  roots  are  seen  at  a  time  during  the  process  of 
transplanting,  and  when  a  plant  is  pulled  or  dug  up  in  the 
ordinary  way  a  large  part  of  the  whole  mass  of  roots  is  broken 
off  and  left  behind.  A  few  plants  have  been  carefully  studied 
to  ascertain  the  total  weight  and  length  of  the  roots.  Those  of 
winter  wheat  have  been  found  to  extend  to  a  depth  of  seven 
feet.  By  weighing  the  whole  root  system  of  a  plant,  and  then 
weighing  a  known  length  of  a  root  of  average  diameter,  the 
total  length  of  the  roots  may  be  estimated.  In  this  way  the 
roots  of  an  oat  plant  have  been  calculated  to  measure  about 


FIG.  22.  Fleshy  roots 
of  garden  rhubarb. 
About  one  fifteenth 
natural  size 


THE  ABSORBING  SURFACE  OF  ROOTS 


27 


154  feet;  that  is,  all  the  roots,  if  cut  off  and  strung  together 
end  to  end,  would  reach  that  distance. 

Single  roots  of  large  trees  often  extend  horizontally  to  great 
distances,  but  it  is  not  often  possible  readily  to  trace  the  entire 
depth  to  which  they  extend.  One  of  the  most  notable  examples 
of  an  enormously  developed  root  system  is  found  in  the  mesquite 
of  the  far  Southwest  and  Mexico.  When  this 
plant  grows  as  a  shrub,  reaching  the  height, 
even  in  old  age,  of  only  two  or  three  feet,  it  is 
because  the  water  supply  in  the  soil  is  very 
scanty.  In  such  cases  the  roots  extend  down 
to  a  depth  of  sixty  feet  or  more,  until  they 
reach  water,  and  the  Mexican  farmers  in 
digging  wells  follow  these 
roots  as  guides.  Where  water 
is  more  abundant,  the  mesquite 
forms  a  "good-sized  tree,  with 
much  shorter  roots. 

35.  The  absorbing  surface 
of  roots.  The  soil  roots  of 
inost  seed  plants  are  provided 
with  a  highly  efficient  means 
for  absorbing  water  in  the 
shape  of  a  coating  of  root  hairs, 
with  which  their  younger  por- 
tions are  thickly  covered. 
Some  idea  of  their  abundance 
may  be  gathered  from  the 
estimate  -  that  on  the  hair- 
bearing  portions  of  the  roots 
of  the  common  pea  about  1437 
hairs  occur  on  every  hundredth  of  a  square  inch  of  surface. 

A  root  hair  is  an  extremely  thin-walled  tube,  springing  from 
an  epidermal  cell,  into  which  it  opens.  The  way  in  which  the 
cells  give  rise  to  hairs  is  well  shown  in  Figs.  21  and  23. 


— n 


FIG.  23 

A,  a  very  young  root  hair;  B,  an  older 
one  (both  greatly  magnified) ;  e,  cells 
of  the  epidermis  of  the  root ;  n,  nu- 
cleus; s,  watery  cell  sap;  p,  proto- 
plasm lining  the  cell  wall.  —  After 
Frank 


28  ROOTS 

Most  water  roots  are  destitute  of  root  hairs,  and  absorb  water 
through  the  general  epidermal  surface  of  their  younger  portions. 

Aerial  roots,  like  those  shown  in  Fig.  13,  are  in  many  cases 
provided  with  an  external  absorbent  layer  of  spongy  tissue,  by 
means  of  which  they  retain  some  of  the  water  which  trickles 
down  them  during  rains.  This  stored  moisture  they  gradually 
give  up  to  the  plant. 

36.  Absorption  of  water  by  roots.    Just  how  much  water 
some  kinds  of  plants  give  off  (and  therefore  absorb)  per  day 
will  be  discussed  when  the  uses  of  the  leaf  are  studied.    For 
the  present  it  is  sufficient  to  state  that  even  an  annual  plant 
during  its  lifetime  absorbs  through  the  roots  very  many  times 
its  own  weight  of  water.    Grasses  have  been  known  to  take 
in  their  weight  of  water  in  every  twenty-four  hours  of  warm, 
dry  weather.    This  absorption  in  most  soil  roots  takes  place 
mainly  through  the  root  hairs.    Their  walls  are  extremely  thin, 
and  have  no  holes  or  pores  visible  under  even  the  highest 
power  of  the  microscope,  yet  the  water  of  the  soil  penetrates 
very  rapidly  to  the  interior  of  the  root  hairs.    The  soil  water 
brings  with  it  all  the  substances  which  it  can  dissolve  from  the 
earth  about  the  plant;  and  the  closeness  with  which  the  root 
hairs  cling  to  the  particles  of  soil,  as  shown  in  Figs.  9  and  21, 
must  cause  the  water  which  is  absorbed  to  contain  more  foreign 
matter  than  underground  water  in  general  does,  particularly 
since  the  roots  give  off  enough  weak  acid  from  their  surface  to 
corrode  the  surface  of  stones  which  they  enfold  or  cover. 

37.  Substances  required  by  the  plant  for  nutrition.    Ordinary 
seed  plants  require  for  their  nutrition  ten  of  the  chemical  ele- 
ments.   By  far  the  greater  part  of  the  weight  of  the  plant  body 
is  usually  due  to  compounds  of  carbon,  hydrogen,  oxygen,  and 
nitrogen.    Besides  these  there  are  present  the  six  elements,— 
sulphur,  phosphorus,  potassium,  calcium,  magnesium,  and  iron. 
In  ordinary  green  meadow  grass  there  is  about  80  per  cent  of 
water  and  20  per  cent  of  dry  matter.    On  drying  the  grass  into 
hay  and  then  burning  the  latter,  some  2  per  cent  of  ash  will  remain, 


SAP  PRESSURE 


29 


and  in  this  will  be  found  the  six  elements  —  sulphur,  phos- 
phorus, potassium,  calcium,  magnesium,  and  iron  —  in  the  form 
of  incombustible  salts  (sulphates,  phosphates,  and  so  on). 

The  plant  gets  its  carbon  and  oxygen  from  the  air,  as  will 
be  explained  in  Chapter  xn.  Deprived  of  air,  all  green  plants 
soon  die.  The  hydrogen  is  obtained 
from  water. 

The  importance  of  the  six  ash- 
forming  constituents  mentioned 
above  is  most  readily  studied  by 
means  of  water  cultures  in  which 
plants  are  grown  with  suitable 
proportions  of  dissolved  salts.  If 
any  one  of  the  six  elements  is 
omitted  from  a  solution,  the 
plants  grown  in  it  are  dwarfish 
and  unhealthy. 

Ordinary  soil  water  contains 
sufficient  salts  in  solution  for  the 
nutrition  of  plants,  but  not  always 
enough  to  stimulate  rapid  growth. 

38.  Sap  pressure.    Not  only 
does  much  water  gain  admission     FIG.  24.  Apparatus  to  measure 
to  the  plant  through  the  roots,  sap  Pressure 

Vmf  nnrlpr  nrrlirmrv  rirrnrrmran^ps  7''  large  tube  fastened  to  the  stumP  of 

the  dahlia  stem  by  a  rubber  tube  ; 

it   is    found    forcing    its    way    011,      r,  r,  rubber  stoppers ;    t,  bent  tube 

into,  and  through  the  stem  (for  — ?, 7S£i  differ 
explanation  see  Sees.  48-51).  The  Sachs 

force  called  sap  pressure  with  which  the  upward-flowing  current 
of  water  presses  may  be  estimated  by  attaching  a  mercury  gauge 
to  the  root  of  a  tree  or  the  stem  of  a  small  sapling.  This  is  best 
done  in  early  spring  after  the  thawing  of  the  ground,  but  before 
the  leaves  have  appeared.  The  experiment  may  also  be  per- 
formed indoors  upon  almost  any  plant  with  a  moderately  firm 
stem,  through  which  the  water  from  the  soil  rises  freely. 


30  ROOTS 

A  dahlia  plant  or  a  tomato  plant  answers  well,  though  the 
sap  pressure  from  one  of  these  will  not  be  nearly  as  great  as  that 
from  a  larger  shrub  or  a  tree  growing  out  of  doors.  In  Pig.  24 
the  apparatus  is  shown  attached  to  the  stem  of  a  dahlia.  The 
difference  of  level  of  the  mercury  in  the  bent  tube  serves  to 
measure  the  pressure.  For  every  foot  of  difference  in  level  there 
must  be  a  pressure  of  nearly  six  pounds  per  square  inch  on  the 
stump  at  the  base  of  the  tube  T.1 

A  black  birch  root  tested  at  the  end  of  April  has  given  a 
sap  pressure  of  thirty-seven  pounds  to  the  square  inch.  This 
would  sustain  a  column  of  water  about  eighty-six  feet  high. 

39.  Root  absorption  and  temperature  of  the  soil.  The  tem- 
perature of  roots  and  the  earth  about  them  has  much  to  do 
with  the  rate  at  which  they  absorb  water.    Some  plants  can 
absorb  it  at  temperatures  as   low  as    25°  F.  (— 4°  C.),  while 
others  cannot  do  so  at  any  temperature  below  39°  F.  (4°  C.). 
This  fact  of  the  power  to  get  water  from  the  soil  ceasing  at  tem- 
peratures in  the  neighborhood  of  the  freezing  point  has  most 
important  consequences,  since  it  implies  that  a  plant  may  die 
for  lack  of  water  with  its  roots  immersed  in  cold,  wet  soil. 
Hence  the  parched  appearance  often  noticed  in  leaves  killed 
by  frost. 

40.  Movements  of  young  roots.    The  fact  that  roots  usually 
grow  downward  is  so  familiar  that  we  do  not  generally  think  of 
it  as  a  thing  that  needs  discussion  or  explanation.    Since  they 
are  pretty  flexible,  it  may  seem  as  though  young  and  slender 
roots  merely  hung  down  by  their  own  weight,  like  so  many  bits 
of  wet  cotton  twine.    But  the  root  of  a  young  Windsor  bean 
seedling  or  of  a  sprouting  pea  will  force  itself  down  into  mer- 
cury.   By  comparing  the  weights  of  equal  bulks  of  mercury  and 
Windsor  bean  roots,  it  is  found  that  the  mercury  is  about  four- 
teen times  as  heavy  as  the  substance  of  the  roots.    Evidently, 
then,  the  submerged  part  of  the  root  must  have  been  held  under 
by  a  force  about  fourteen  times  its  own  weight. 

1  For  a  more  accurate  method  see  Handbook- 


GEOTROPISM  31 

A  more  accurate  measurement  of  the  force  exerted  by  the 
root  may  be  made  by  confining  it  so  it  cannot  bend,  and  letting 
it  push  down  on  a  spring.  In  this  way  it  is  found  that  the  root 
of  the  Windsor  bean  can  push  with  a  pressure  of  about  ten 
ounces. 

Making  fine  equidistant  cross  marks  with  ink  along  the  upper 
and  the  lower  surface  of  a  root  that  is  about  to  bend  downward 
at  the  tip  readily  shows  that  those  of  the  upper  series  soon 
come  to  be  farther  apart,  —  in  other  words,  that  the  root  is 
forced  to  bend  downward  ty  the  more  rapid  growth  of  its  upper 
as  compared  with  its  under  surface. 

41.  Geotropism.  The  property  which  plants  or  their  organs 
manifest,  of  assuming  a  definite  direction  with  reference  to  grav- 
ity,1 is  called  geotropism.  When,  as  in  the  case  of  the  primary 
root,  the  effect  of  gravity  is  to  make  the  part,  if  unobstructed, 
turn  or  move  downward,  we  say  that  the  geotropism  is  positive. 
If  the  tendency  is  to  produce  upward  movement,  we  say  that 
the  geotropism  is  negative;  if  horizontal  movement,  that  it  is 
lateral.  It  was  stated  in  the  preceding  section  that  the  direct 
cause  of  the  downward  extension  of  roots  is  unequal  growth. 
We  might  easily  suppose  that  this  unequal  growth  is  not  due 
to  gravity,  but  to  some  other  cause.  To  test  this  supposition, 
the  simplest  plan,  if  it  could  be  carried  out,  would  be  to  remove 
the  plants  studied  to  some  distant  region  where  gravity  does  not 
exist.  This  of  course  cannot  be  done,  but  we  can  easily  turn  a 
young  seedling  over  and  over  so  that  gravity  will  act  on  it  now 
in  one  direction,  now  in  another,  and  so  leave  no  more  impres- 
sion than  if  it  did  not  act  at  all.  Or  we  can  whirl  a  plant  so 
fast  that  not  only  is  gravity  done  away  with,  but  another  force 
is  introduced  in  its  place.  If  a  vertical  wheel,  like  a  carriage 
wheel,  were  provided  with  a  few  loosely  fitting  iron  rings  strung 
on  the  spokes,  when  the  wheel  was  revolved  rapidly  the  rings 
would  all  fly  out  to  the  rim  of  the  wheel.  So  in  Fig.  25  it  will 

1  Gravity  means  the  pull  which  the  earth  exerts  upon  all  objects  on  or 
near  its  surface, 


32 


ROOTS 


be  noticed  that  the  growing  tips  of  the  roots  of  the  sprouting 
peas  point  almost  directly  outward  from  the  center  of  the  disk 

on  which  the  seedlings  are  fastened. 
In  this  case  the  so-called  "centrif- 
ugal force"  due  to  the  rotation  of 
the  wheel  is  sufficient  wholly  to 
overcome  geotropism. 

42.  Direction  taken  by  secondary 
roots.  As  the  student  has  already 
noticed  in  the  seedlings  which  he 
has  studied,  the  branches  of  the 
primary  root  usually  make  a  con- 
siderable angle  with  it  (Fig.  2). 
Often  they  run  out  for  long  dis- 


FIG.  25.   Sprouting   peas   on   a 
rapidly  whirling  disk 


The  youngest  portions  of  the  roots 

all  point  directly  away  from  the   tances  almost  horizontally.    This  is 
axis  about  which  they  were  re-   especially  common  in  the  roots  of 

volved.  — After  Detmer 

forest  trees,  above  all  m  cone-bearing 

trees,  such  as  pines  and  hemlocks  (Fig.  26).    This  horizontal, 
or  nearly  horizontal,  position  of  large  secondary  roots  is  the 


FIG.  26.    Roots  of  a  white  pine 

most  advantageous  arrangement  to  make  them  useful  in  stay- 
ing or  guying  the  stem  above  to  prevent  it  from  being  blown 
over  by  the  wind. 


ADAPTATIONS   TO   CONDITIONS  OF  LIFE  33 

43.  Fitness  of  the  root  for  its  position  and  work.  The  dis- 
tribution of  material  in  the  woody  roots  of  trees  and  shrubs  and 
their  behavior  in  the  soil  show  many  adaptations  to  the  condi- 
tions by  which  the  roots  are  surrounded.  The  growing  tip  of 
the  root,  as  it  pushes  its  way  through  the  soil,  is  exposed  to 
bruises ;  but  these  are  largely  warded  off  by  the  root  cap.  The 
tip  also  shows  a  remarkable  sensitiveness  to  contact  with  hard 
objects,  so  that  when  touched  by  one  it  swerves  aside  and  thus 
finds  its  way  downward  by  the  easiest  path.  Eoots  with  an 
unequal  water  supply  on  either  side  grow  toward  the  moister 
soil ;  when  unequally  heated  they  grow  in  the  direction  of  the 
most  desirable  temperature,  and  they  usually  grow  away  from 
the  light.  Eoots  are  very  tough,  because  they  need  to  resist 
strong  pulls,  but  not  as  stiff  as  stems  and  branches  of  the  same 
size,  because  they  do  not  need  to  withstand  sidewise  pressure, 
acting  from  One  side  only.  The  corky  layer  which  covers  the 
outsides  of  roots  is  remarkable  for  its  power  of  preventing  evapo- 
ration. It  must  be  of  use  in  retaining  in  the  root  the  moisture 
which  otherwise  must  be  lost  on  its  way  from  the  deeper  root- 
lets (which  are  buried  in  damp  soil),  through  the  upper  portions 
of  the  root  system,  about  which  the  soil  is  often  very  dry. 


CHAPTER  V 

SOME   PROPERTIES   OF   CELLS   AND   THEIR   FUNCTIONS   IN 

THE  ROOT 

44.  Definition  of  cell.    This  is  not  the  best  place  to  consider 
the  nature  of  cells  in  much  detail  (see  Chapter  xvm) ;  but  some 
of  the  facts  learned  in  Chapter  iv  cannot  be  understood  with- 
out a  few  words  of  explanation  of  cell  structure  and  functions. 

Protoplasm  is  the  nitrogenous  living  substance  of  which 
the  most  rapidly  growing  parts  of  plants  are  mainly  composed. 
The  activities  of  the  plant  are  due  to  the  peculiar  qualities  and 
powers  of  protoplasm.  A  cell  is  a  unit  of  protoplasm,  called 
a  protoplast.  The  protoplast  of  plants  is  usually  inclosed  hi  a 
case  or  covering  whose  walls  (cell  walls)  are  composed  of  a  sub- 
stance known  as  cellulose.  Each  protoplast  usually  contains  a 
single  denser  protoplasmic  structure,  called  the  nucleus. 

In  form  and  size  cells  vary  greatly.  Those  of  the  root  hair 
(Fig.  23)  are  good  examples  of  the  slender,  thread-like  form ; 
those  of  Fig.  27  well  illustrate  forms  commonly  assumed  when 
cells  are  pressed  upon  by  others  on  all  sides,  as  they  usually 
are  in  the  interior  portions  of  the  organs  of  higher  plants. 

45.  Growth  and  reproduction.   The  most  remarkable  property 
of  cells  is  their  power  of  growth  and  reproduction.    Growth 
results  not  only  from  an  increase  in  the  size  of  cells  but  also 
in  their  number  as  a  result  of  cell  division.    This  is  the  separa- 
tion of  a  protoplast,  generally  into  two  independent  protoplasts 
or  daughter  cells,  and  is  the  fundamental  cause  of  all  growth 
and   development.     The    full-grown   seed    plant,   composed    of 
millions  of  cells,  arises  from  the  embryo  (with  perhaps  only  a 
few  thousand),  which  had  its  beginning  in  a  single  cell.     Cell 
division  is  preceded  by  division  of  the  nucleus  (Fig.  170). 

34 


IRRITABILITY 


35 


Reproduction,  or  the  formation  of  new  organisms  similar  to 
the  parents,  is  possible  only  for  protoplasm,  not  for  any  other 
known  substance. 

46.  Irritabil- 
ity. Another 
characteristic  of 
protoplasm  is  its 
irritability.  By 
this  is  meant  its 
power  of  re- 
sponding in 
some  way  to  an 
application  of 
energy  which 
serves  as  a  stim- 
ulus. A  famous 
plant  physiolo- 
gist1 has  illus- 
trated the  matter 
very  simply 
thus :  A  wound- 
up alarm  clock, 
which  is  not 

going,  is  given  a  ^  cells  from  ovu]e  (x  340) .  ^  celis  fr0m  an  ovule  further 
developed  (x  340) ;  C,  J),  cells  from  pulp  of  fruit  (x  110) ; 
n,  nucleus;  p,  protoplasm;  s,  cell  sap.  —  After  Prantl 


n-' 


FIG.  27.   Protoplasts  in  ovule  and  fruit  of  snowberry 
(Symphoricarpus  racemosus) 


In  the  young  and  rapidly  growing  cells  A  and  B  the  cell 
sap  is  not  present,  or  present  only  in  small  quantities, 
while  in  the  older  cells  C  and  D  it  occupies  a  large  por- 
tion of  the  interior  of  the  cell 


shake  (stimulus), 

which  starts  the 

clock,  and  after 

an  interval  of 

time    (latent 

period)  rings  the  alarm  (result).    The  sensitiveness  of  the  clock 

to  any  jar  which  sets  it  going  corresponds  to  the  irritability  of 

living  protoplasm.    This  extremely  delicate  responsiveness  may 

be  manifested  in  a  simple  cell  or  in  an  organ  or  entire  plant 

composed  of  multitudes  of  cells. 

1  Professor  W.  Pfeffer,  of  Leipzig,  Germany. 


36  SOME  PROPERTIES  OF   CELLS 

Some  of  the  most  important  stimuli  which  call  out  manifes- 
tations of  irritability  in  protoplasm  are  heat,  light,  electricity, 
gravity,  pressure  of  external  objects,  and  contact  with  substances 
which  act  chemically  on  the  protoplasm.  Many  instances  of 
irritability  will  come  up  in  later  chapters.  A  notable  example 
of  response  to  a  stimulus  is  the  beginning  of  germination  in 
seeds  subjected  to  a  suitable  degree  of  heat  in  presence  of 
moisture. 

The  ways  in  which  the  responses  to  stimulation  may  show 
themselves  are  very  numerous,  and  the  same  individual  or  organ 
may  be  favorably  affected  by  a  certain  amount  of  a  given  stimu- 
lus and  unfavorably  by  a  greater  amount  of  the  same  stimulus. 
Every  one  has  had  the  experience  of  drawing  near  to  a  moder- 
ately heated  stove  in  cold  weather  and  then  retreating  from  it 
when  the  fire  grew  too  hot.  So,  too,  certain  microscopic  uni- 
cellular plants,  living  in  water,  move  toward  the  light  until  it 
reaches  a  certain  intensity,  but  when  that  intensity  is  passed, 
they  move  in  the  opposite  direction,  toward  the  dark. 

47.  Selective  absorption.  Another  extremely  important  power 
of  live  protoplasm  is  that  of  selective  absorption.    By  this   is 
meant  the  ability  to  take  up  from  liquids  or  gases  certain  sub- 
stances  and  leave  unabsorbed  other  elements  or  compounds 
which  are  also  present. 

Thus  plants  of  two  different  species,  both  growing  in  the 
same  soil,  usually  take  from  it  very  various  amounts  or  kinds 
of  mineral  matter.  For  instance,  barley  plants  in  flower  and 
red  clover  plants  in  flower  contain  about  the  same  proportion 
of  mineral  matter  (left  as  ashes  after  burning).  But  the  clover 
contains  5|  times  as  much  lime  as  the  barley,  and  the  latter 
contains  about  18  times  as  much  silica  as  the  clover.  This  dif- 
ference must  be  due  to  the  selective  action  of  the  protoplasm 
in  the  absorbing  cells  of  the  roots. 

48.  Osmosis.    The  process  by  which  two  liquids  of  different 
densities  separated  by  membranes  pass  through  the  latter  and 
mingle,  as  soil  water  does  with  the  liquid  contents  of  root  hairs, 


OSMOSIS 


37 


is  called  osmosis.  It  is  readily  demonstrated  by  experiments 
with  thin  animal  or  vegetable  membranes.  For  instance,  when 
prunes,  raisins,  or  other  dried  fruit,  are  put  in  water  to  soak, 
water  penetrates  the  outer  skin  and  swells  the  seed  or  fruit, 
while  some  of  the  material  from  within  comes  out  through  the 
skin  and  flavors  or  discolors  the  water.  If  whole  cranberries, 
cherries,  or  plums  are  put  in- 
to boiling  sirup,  a  similar  ex- 
change takes  place,  but  in  this 
case  the  fruit  is  shriveled. 

A  still  better  experiment  is 
that  with  an  egg  from  which 
a  bit  of  the  shell  has  been 
chipped  away  at  the  bottom, 
arranged  as  shown  in  Fig.  28. 
The  entrance  of  water  is  shown 
by  the  rise  of  some  of  the  con- 
tents of  the  egg  in  the  tube. 

49.  Inequality  of  osmotic 
exchange.  The  nature  of  the 
two  liquids  separated  by  any 

given    membrane    determines       FlG  28    Egg  on  beaker  of  water, 
in  which  direction  the  greater  to  show  osmosis 

flow    shall    take    place    unless    The  tube  is  cemented  to  the  eggshell,  into 


which  it  opens.  At  the  bottom  a  large 
piece  of  the  shell  has  been  chipped 
away,  leaving  the  thin  skin  which  lines 
the  egg  in  contact  with  the  water  in  the 
beaker 


what  would  naturally  be  the 
direction  of  flow  is  overruled 
by  the  selective  action  of  liv- 
ing protoplasm. 

If  one  of  the  liquids  is  pure  water  and  the  other  is  water 
containing  solid  substances  dissolved  in  it,  the  greater  flow  of 
liquid  will  be  away  from  the  pure  water  into  the  solution,  and 
the  stronger  or  denser  the  latter,  the  more  unequal  will  be  the 
flow.  This  principle  is  well  illustrated  by  the  egg-osmosis 
experiment.  Another  important  principle  is  that  substances 
which  readily  crystallize  and  are  easily  soluble,  like  salt  or 


38 


SOME  PROPERTIES  OF  CELLS 


sugar,  pass  rapidly  through  membranes,  while  jelly-like  sub- 
stances, like  white  of  egg,  can  hardly  pass  through  them 
at  all. 

50.  Study  of  osmotic  action  of  living  protoplasm ;  plasmol- 
ysis.  The  obvious  parts  of  most  living  and  growing  plant  cells 
are  a  cell  wall,  which  is  a  skin  or  inclosure  made  of  cellulose,  and 
the  living,  active  cell  contents,  or  protoplasm  (Sec.  44).  Every 
one  is  familiar  with  cellulose  in  various  forms,  one  of  the  best 
examples  being  that  afforded  by  clean  cotton.  It  is  a  tough, 
white,  or  colorless  substance,  and  chemically  rather  inactive. 

Often,  in  living  cells, 
the   spaces  between 
'J    strands  and  protoplas- 
—p    mic  lining   are    filled 
s    with  a  watery  liquid 
called  the  cell  sap. 

The  action  of  living 
protoplasm  in  control- 
ling osmosis  is  well 
shown  by  the  process 
known  as  plasmolysis. 
If  thin-walled  cells 


A  B 

FIG.  29.    Cells  from  root  of  Indian  corn 

A,  in  natural  condition ;  D,  plasmolyzed  in  5  per  , 

cent  solution  of  potassium  nitrate;  w,  cell  wall;  il(lu  m> 

p,  denser  part  of  protoplasm ;  s,  cell  sap.    Much  such   as   tllOSC    of    O116 
magnified.  —  After  Pfeffer  .  ,,  -, 

or  the  pond  scums,  are 

put  into  a  salt  solution,  the  cell  contents  will  shrink  away  from 
the  cell  wall  (Fig.  168,  B)  because  the  direction  of  flow,  toward 
the  denser  liquid,  draws  water  out  of  the  cell.  Repeating  the 
experiment  with  a  cell  which  has  been  killed  by  a  few  minutes' 
immersion  in  a  poisonous  solution  (e.g.  of  chromic  acid)  shows 
no  plasmolysis. 

So,  too,  slices  of  a  red  beet  impart  little  color  to  water  in  which 
they  are  placed,  but  after  the  cells  are  killed  by  boiling  the  color 
comes  out  freely. 


BEHAVIOR  OF  ROOTS  DUE  TO  IRRITABILITY      89 

51.  Osmosis  in  root  hairs.   The  soil  water,  practically  identi- 
cal with  ordinary  spring  or  well  water,  is  separated  from  the 
more  or  less  sugary  or  mucilaginous  sap  inside  of  the  root  hairs 
only  by  their  delicate  cell  walls,  lined  with  a  thin  layer  of  pro- 
toplasm.   This  soil  water  will  pass  rapidly  into  the  plant,  while 
very  little   of  the   sap  will   come   out.    The   selective   action, 
which  causes  the  flow  of  liquid  through  the  root  hairs  to  be 
almost  wholly  inward,  is  due  to  the  living  layer  of  protoplasm, 
which  covers  the  inner  surface  of  the  cell  wall  of  the  root  hair. 
Traveling  by  osmotic  action  from  cell  to  cell,  a  current  of  water 
derived  from  the  root  hairs  is  forced  up  through  the  roots  and 
into  the  stem,  somewhat  as  the  contents  of  the  egg  was  forced 
up  into  the  tube  shown  in  Fig.  28. 

But  there  is  this  important  difference  in  the  two  cases,  that 
while  the  process  in  the  tube  was  all  due  to  the  impulse  received 
at  the  start  from  the  egg  membrane,  in  the  plant  stem  the  origi- 
nal pressure  due  to  osmosis  in  the  root  hairs  may  be  affected  by 
osmosis  in  countless  thousands  of  cells  higher  up. 

52.  Behavior  of  roots  due  to  irritability.    In  Chapter  iv  a 
little  was  said  about  the  geotropism  of  roots,  their  tendency  to 
put  themselves  into  the  most  favorable  conditions  as  regards 
moisture,  heat,  and  light,  and  their  manner  of  avoiding  obstacles. 
All  these  actions  are  manifestations  of  irritability. 

The  subject  of  geotropism  of  roots  is  a  very  complicated  one, 
but  it  seems  pretty  certain  that  gravity  somehow  acts  as  a  stim- 
ulus on  the  sensitive  cells  of  the  root  tip,  this  stimulus  is  trans- 
mitted to  the  cells  of  the  most  rapidly  growing  portion  of  the 
root  (a  little  farther  back),  unequal  groivth  of  the  upper  and 
under  cells  of  this  portion  follows,  and  so  the  root  is  bent,  if  its 
position  is  not  vertical  in  the  beginning. 

Moisture  and  heat  (in  the  case  of  Indian  corn  up  to  99.5°  F. 
or  37.5°  C.)  are  favorable  to  the  growth  of  roots,  and  so  as  stimuli 
produce  growth  toward  the  source  of  moisture  or  heat,  while 
light  is  usually  slightly  unfavorable  and  therefore  generally 
results  in  growth  of  the  root  toward  darkness. 


CHAPTER  VI 
STEMS 

53.  Nature  of  the  stem.  The  work  of  taking  in  the  raw 
materials  which  the  plant  makes  into  its  own  food  is  done 
mainly  by  the  roots  and  the  leaves.  These  raw  materials  are 
taken  from  earth,  from  water,  and  from  the 
air  (see  Chapter  xn).  The  stem  is  that  part  or 
organ  of  the  plant  which  serves  to  bring  roots 
and  leaves  into  communication  with  each 
other.  In  most  seed  plants  the  stem  also 
serves  the  important  purpose  of  lifting  the 
leaves  up  into  the  sunlight,  where  they  can 
best  do  their  special  work. 

The   student   has   already,  in  Chapter  in, 
learned  something  of  the  development  of  the 
stem  and  the  seedling ;  he  has  now  to  study 
the    external   and   internal   structure  of  the 
mature  stem.    Much  in  regard  to  this  struc- 
bsc      ture   can    be   learned   most  easily  from  the 
examination   of  twigs   and  branches   of   our 
common  forest  trees  in  their  winter  condition. 
FIG.  30.  A  quickly        54.  Position  of  leaf  buds.   The  winter  buds 
grown  twig  of   of  most  of  our  trees  and  shrubs  are  formed  at 
cherry    with    p0^nts  on  the  twig  ;ust  above  the  origins  of 
lateral  and  termi-    J  &   J 

nal  buds  in  Oc-   tne  leafstalks,  as  shown  in  Fig.  79.    After  the 

tober  fall  of  the  leaves  the  buds  by  their  positions 

6  sc,  bud-scale  scars,    indicate  where  the  leaves  were  formerly  at- 

All  above  these          ,      ,      m,  ,  i  •  •  i      i 

scars  is  the  growth   tached.    They  may  be  arranged  in  pairs,  a  bud 

of  the  spring  and   on  one  si(je  of  the  stem  and  its  mate  exactly 

summer  of  the  .  „  .     ,  , 

same  year  opposite,  or  they  may  form  a  spiral  around 

40 


METHODS  OF  BRANCHING 


41 


the  stem,  as  shown  in  Fig.  30.  Since  every  leaf  bud  —  that  is, 
every  bud  which  contains  rudimentary  leaves  —  will,  if  success- 
ful, grow  into  a  branch,  the  position  of 
the  buds  is  most  important  in  deter- 
mining the  shape  of  the  tree. 

55.  Opposite  branching.  Trees  with 
opposite  leaves  and  buds  show  a  tend- 
ency to  form  twigs  in  four  rows  about 
at  right  angles  to  each  other  along  the 
sides  of  the  branch,  as  shown  in  Fig.  31. 
This  arrangement  will  not  usually 
be  perfectly  carried  out,  as  most  of  the 
buds  never  grow, 
since  they  are 
shaded  and  starved, 
or  some  may  grow 
much  faster  than 
others  and  so  make 
the  plan  of  branch- 
ing less  evident 
than  it  would  be  if  all  grew  alike. 

56.  Alternate  branching.    In  trees  like 
the  beech  the  twigs  will  be  found  to  be 
arranged  in  a  more  or  less  regular  spiral 
line  about  the  branch.    This,  which  is  known 
as  the  alternate  arrangement  (Fig.  32),  is 
more  commonly  met  with  in  trees  and  shrubs 
than  the  opposite  arrangement.    It  admits  of 
many  varieties,  since  the  spiral  may  wind 
more  or  less  rapidly  round  the  stem.    In  the 
FIG.  32.   Alternate   apple,  pear,  cherry,  poplar,  oak,  and  walnut, 
branching  m  a  very   one  passes  OVer  five  spaces  before  coming  to 
young  apple  tree  ,      „      ,  .  ,     .  -i    •       -i    • 

a  leaf  which  is  over  the  first,  and  in  doing 

this  it  is  necessary  to  make  two  complete  turns  around  the 
stem  (Fig.  100). 


FIG.  31.  Opposite  branch- 
ing in  a  very  young  sap- 
ling of  ash 


42 


STEMS 


57.  Growth  of  the  terminal  bud.  In  some  trees  the  termi- 
nal bud  from  the  outset  keeps 
the  lead  and  produces  a  slen- 
der, upright  tree  (Fig.  33),  as 
in  the  pines,  spruces,  and  firs. 

In  such  trees  as  the  apple 
and  many  oaks  the  terminal 
bud  has  no  preeminence  over 
others,  and  the  form  of  the 
tree  is  round-topped  and 
spreading  (Fig.  34).  Most 
forest  trees  are  intermediate 
between  these  extremes. 

Branches  owe  their  char- 
acteristics to  several  factors. 
Most  of  our  trees  and  larger 
shrubs  make  a  definite  annual 
growth,  with  the  buds  ripened 
before  the  coming  of  winter 
(Fig.  79).  In  these  the  ter- 
minal bud  is  likely  to  grow 
and  continue  the  branch. 
Such  shrubs  and  trees  as  the 
raspberry  and  blackberry,  the 
sumach  and  the  ailanthus, 
make  an  indefinite  annual 
growth,  that  is,  the  tips  of  the 
branches  are  usually  killed 
by  frost,  and  so  the  tree  forks 
often.  Terminal  flower  buds 
(Figs.  36,  37)  also  cause  fork- 
ing and  allow  the  tree  to  form 

i  O 

FIG.  33.   California  giant    redwoods      no  long,  straight  branches. 

),  illustrating  upright  growth  jf    the    terminal    buds    of 

After  J.  H.  White  branches  keep  the  lead  of  the 


FORMS  OF  TREES 


43 


FIG.  34.    An  American  elin,  illustrating  spreading  growth 

lateral  ones,  but  the  latter  are  numerous  and  most  of  those 
which  survive  grow  into  slender  twigs,  the  delicate  spray  of 
the  elm  and  many  birches  is  produced  (Fig.  38). 

The  general  effect  of  the  branching  depends  much  upon  the 
angle  which  each  branch  or  twig  forms  with  that  one  from  which 


44 


STEMS 


it  springs.  The  angle  may  be  quite  acute,  as  in  the  birch 
(Fig.  38) ;  or  more  nearl)7  a  right  angle,  as  in  the  ash  (Fig.  31). 
The  inclination  of  lateral  branches  is  due  to  geotropism,  just 
as  is  that  of  the  branches  of  primary  roots.  The  vertically 
upward  direction  of  the  shoot  which  grows  from  the  terminal 
bud  is  also  due  to  geotropism,  which,  however,  in  the  shoot,  is 
exactly  opposite  to  that  in  the  root. 

This  is  really  only  a  brief  way  of  saying  that  the  growing  tip 
of  the  main  stem  of  the  tree,  or  of  any  branch,  is  made  to  take 
and  keep  its  proper  direc- 
tion, whether  vertically 
upward  or  at  whatever 
angle  is  desirable  for  the 
tree,  by  the  steering  action 
of  gravity.  After  growth 
has  ceased  this  steering 
action  can  no  longer  be 
exerted,  and  so  a  tree 
that  has  been  bent  over 
—  as,  for  instance,  by  a 
heavy  load  of  snow  — 
cannot  right  itself  unless 
it  is  elastic  enough  to 

spring  back  when  the  load  is  removed.  The  tip  of 
the  trunk  and  of  each  branch  can  grow  and  thus 
become  vertical,  but  the  old  wood  cannot  do  so. 

58.  Thorns  as  branches.  In  many  trees  some 
branches  show  a  tendency  to  remain  dwarfish 
and  incompletely  developed.  Such  imperfect 
branches  may  form  thorns,  as  in  the  familiar  wild 
crab-apple  trees  and  in  the  pear  trees  which  occur  in  old  pas- 
tures in  the  northeastern  states.  In  the  honey  locust  very  for- 
midable brandling  thorns  spring  from  adventitious  or  dormant 
buds  on  the  trunk  or  limbs.  They  sometimes  show  their  true 
nature  as  branches  by  bearing  leaves  (Fig.  35). 


FIG.  35.    Leaf-bearing 
thorn  of  honey  locust 


TREES,  SHRUBS,  AND  HERBS 


45 


59.  Trees,  shrubs,  and  herbs.  Plants  of  the  largest  size,  with 
a  main  trunk  of  a  woody  structure,  are  called  trees.  Shrubs 
differ  from  trees  in  their  smaller  size,  and  generally  in  having 
several  stems  which  proceed  from  the  ground  or  near  it,  or  in 
having  much-forked  stems.  The  witch-hazel,  the  dogwoods,  and 


FIG.  30.   Tip  of  a  branch  of  magnolia,  illustrating  forking  due  to 
terminal  flower  buds 

A,  oldest  flower-bud  scar;  B,  C,  D,  scars  of  successive  seasons  after  A;  L,  leaf 
buds ;  F,  flower  buds 

the  alders,  for  instance,  are  most  of  them  classed  as  shrubs  for 
this  reason,  though  in  height  some  of  them  equal  the  smaller 
trees.  Some  of  the  smallest  shrubby  plants,  like  the  dwarf  blue- 
berry, the  wintergreen,  and  the  trailing  arbutus,  are  only  a  few 


46 


STEMS 


inches  in  height,  but  are  ranked  as  shrubs  because  their  woody 
stems  do  not  die  to  the  ground  in  winter. 

Herbs  are  plants  whose  stems  above  ground  die  every  winter. 

60.  Annual,  biennial, 
and  perennial  plants. 
Annual  plants  are  those 
which  live  but  one  year, 
biennials  those  which 
live  two  years  or  nearly  so. 

Some  winter  annuals 
do  not  flower  until 
their  second  summer. 
This  is  true  of  the  even- 
ing primrose  and  the 
fringed  gentian,  and  of  winter  wheat  and  rye 
among  cultivated  plants. 

Perennial  plants  live  for  a  series  of  years.  Many  kinds  of 
trees  last  for  centuries.  The  California  giant  redwoods,  or 
Sequoias  (Fig.  33),  which  reach  a  height  of  over  300  feet  under 


\ 


FIG.  37.   A  portion  of  a 
branch  of  Fig.  36 

Natural  size 


„.  _„.    Twigs  and 
branches  of  the 
birch 


v-^J      7 

favorable  circumstances,  live  nearly  2000  years ;  and  some 
enormous  cypress  trees  found  in  Mexico  were  thought  by  Pro- 
fessor Asa  Gray  to  be  from  4000  to  5000  years  old. 


48 


STEMS 


•4w 


61.  Climbing  and  twining  stems.1  Since  it  is  essential  to  the 
health  and  rapid  growth  of  most  plants  that  they  should  have 
free  access  to  the  sun  and  air,  it  is  not  strange  that  many  should 
resort  to  special  devices  for  lifting  themselves  above  their  neigh- 
bors. In  tropical  forests,  where  the  darkness  of  the  shade  any- 
where beneath  the  tree  tops  is  so  great  that  few  flowering 
plants  can  thrive  in  it,  the  climbing  plants,  or  lianas  (Fig.  39), 
often  run  like  great  cables  for  hundreds  of  feet  before  they  can 
emerge  into  the  sunshine  above.  In  temperate  climates  no  such 
remarkable  climbers  are  found,  but  many  plants  raise  themselves 
for  considerable  distances.  The  principal  means  by  which  they 

accomplish  this  result  are : 

1.  Producing  roots  at  many  points 
along  the   stem    above    ground  and 
climbing  on  suitable  objects  by.  means 
of  these,  as  in  the  English  ivy  (Fig.  14). 

2.  Laying  hold  of  objects  by  means 
of  tendrils  or  twining  branches  or  leaf- 
stalks, as  shown  in  Figs.  40  and  41. 

3.  Twining  about  any  slender  up- 
right support,  as  shown  in  Fig.  42. 

4.  Clambering    upon    bushes    and 
other  supports  by  means  of  hooked 
prickles,  as  is   done   by  some  roses, 
blackberries,  and  cleavers  (Galium). 

62.  Tendril  climbers.  The  plants 
which  climb  by  means  of  tendrils  are 
important  subjects  for  study.  Con- 
tinued observation  soon  shows  that 
the  tips  of  tendrils'  sweep  slowly  about 
in  a  circular  or  oval  course  until  they  come  in  contact  with  some 
object  around  which  they  can  coil.  After  the  tendril  lias  taken 
a  few  turns  about  its  support,  the  free  part  of  the  tendril  coils 
into  a  spiral  and  thus  draws  the  whole  stem  toward  the  point 
1  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  I,  p.  669. 


FIG.  40.    Coiling  of  a  tendril 
of  bryony 
After  Sachs 


CLIMBERS  AND   TWINERS 


49 


of  attachment,  as  shown  in  Fig.  40.  Some  tendrils  are  modified 
leaves  or  stipules,  as  shown  in  Fig.  98  ;  others  are  modified  stems. 
63.  Irritability  of  tendrils.  The  coiling  of  tendrils  is  due 
to  their  irritability,  aroused  by  the  stimulus  of  contact  with  a 
solid  object.  After  a  latent  period,  varying  with  different  species 
from  a  few  seconds  to  more  than  an  hour,  the  bending  begins. 
It  is  caused  either  by  contraction  of  the  side  in  contact  or  by 
expansion  of  the  opposite  side;  the  exact  mechanism  of  the 
process  is  not  yet  fully  under- 
stood. The  tendrils  of  the  passion- 
flower plant  will  respond  to  the 


FIG.  41.    Coiling  of  petiole  of  dwarf 
nasturtium  ( Tropceolum) 


FIG.  42.  Twining  stem  of  hop 
After  Decaisne 


pressure  of  a  bit  of  thread,  hung  on  the  tendril  and  kept  in 
motion,  whose  weight  is  only  a  few  millionths  of  a  grain. 

64.  Twiners.  Only  a  few  of  the  upper  internodes  of  the  stem 
of  a  twiner  are  concerned  in-  producing  the  movements  of  the 
tip  of  the  stem.  This  is  kept  revolving  in  an  elliptical  or  cir- 
cular path  until  it  encounters  some  roughish  and  not  too  stout 
object,  about  which  it  then  proceeds  to  coil  itself. 

The  movements  of  the  younger  internodes  of  the  stems  of 
twiners  are  among  the  most  extensive  of  all  the  movements 


50 


STEMS 


made  by  plants.  A  hop  vine  which  has  climbed  to  the  top  of 
its  stake  may  sweep  its  tip  continually  around  the  circumfer- 
ence of  a  circle  two  feet  in  diameter,  and  the  common  wax 
plant  (Hoya)  of  the  greenhouses  sometimes  describes  a  five-foot 
circle,  the  tip  moving  at  the  rate  of  thirty-two  inches  per  hour.1 

This  circular  motion  is 
produced  by  unequal 
growth  of  the  two  sides 
of  the  stem.2 

The  direction  in 
which  twiners  coil 
about  a  supporting  ob- 
ject is  almost  always 
the  same  for  each  spe- 
cies of  plant,  but  not 
the  same  for  all  species. 
In  the  hop  it  is  as  shown 
in  Fig.  42,  but  in  many 
plants  the  movement  is 
in  the  reverse  direction. 
65.  Short-stemmed 
plants.  As  will  be 

FIG.  43.   The  dandelion,  a  short-stemmed  plant    shown   later    (Chapter 

xxxiv),  plants  live  sub- 
ject to  a  very  fierce  competition  among  themselves,  and  they  are 
exposed  to  almost  constant  attacks  from  animals. 

While  plants  with  long  stems  find  it  to  their  advantage  to 
reach  up  as  far  as  possible  into  the  sunlight,  the  dandelion,  the 
cinquefoil,  the  white  clover,  some  spurges,  the  knotgrass,  and 
hundreds  of  other  species,  living  in  open  places,  have  found 
safety  in  hugging  the  ground.  The  dandelion,  fall  dandelion, 

1  See  article  on  "  Climbing  Plants,"  by  Dr.  W.  J.  Beal,  in  the  American 
Naturalist,  Vol.  IV,  pp.  405-415. 

2  See  Strasburger,  Noll,  Schenk,  and  Schimper,  Text-Book  of  Botany, 
pp.  257-260,  New  York,  1903. 


SHORT-STEMMED  PLANTS 


51 


shepherd's  purse,  and  the  like,  with  radiating  leaves,  are  known 
as  rosette  plants,  while  those  with  radiating  stems,  like  knotgrass, 


FIG.  45.    Roots,  rootstocks,  and 
leaves  of  Iris 


FIG.  44.   Rootstock  of  cotton  grass  (Eriophorum) 

the  clovers,  and  black  medick  (Medicago) ,  are  known  as  mat  plants. 

Any  plant  which  can  grow  in  safety  under  the  very  feet  of 

grazing  animals  will  be  especially  likely  to  make  its  way  in 


52 


STEMS 


the  world,  since  there  are  many  places  where  it  can  nourish 
while  ordinary  plants  would  be  destroyed.  The  bitter  dandelion, 
which  is  almost  uneatable  for  most  animals  on  account  of  its 
taste,  which  lies  too  near  the  earth  to  be  fed  upon  by  grazing 
animals,  and  which  bears  being  trodden  on  with  impunity,  is  a 
type  of  a  large  class  of  hardy  weeds. 

The  plants  incorrectly  called  "stemless,"  like  the  dandelion 
(Fig.  43)  and  some  violets,  are  not  really  stemless,  but  send  out 


b'. 


FIG.  46.   Rootstock  of 
caladium  (Colocasia) 

b,  terminal  bud;  &',  buds 
arranged  in  circles  where 
bases  of  leaves  were  at- 


FIG.  47.    Part  of  a  potato  plant 


tached ;    s,  scars  left  by    The  dark  tuber  in  the  middle  is  the  one  from  which 
sheathing  bases  of  leaves  the  plant  has  grown 

their  leaves  and  flowers  from  a  very  short  stem  which  hardly 
rises  from  the  surface  of  the  ground. 

66.  Underground  stems.  Stems  which  lie  mainly  or  wholly 
underground  are  of  frequent  occurrence  and  of  many  kinds. 
Some  of  the  simplest  kinds  are  called  rootstocks.  Familiar  ex- 
amples are  those  of  some  mints,  of  bloodroot,  of  Solomon's  seal, 
and  of  many  grasses,  sedges,  and  ferns.  The  real  nature  of  the 
creeping  underground  stem  is  frequently  shown  by  the  pres- 
ence upon  its  surface  of  many  scales,  which  are  reduced  leaves. 


UNDERGROUND  STEMS 


53 


Exterior  view,  and  split  lengthwise.  — 
After  Faguet 


Rootstocks  of  this  sort  often  extend  horizontally  for  long  dis- 
tances in  the  case  of  grasses  like  the  sea  rye  grass  (Plate  I), 

which  roots  itself  firmly  and 
thrives  in  shifting  sand  dunes. 
In  the  stouter  rootstocks,  like 
that  of  the  iris  (Fig.  45)  and 
the  caladium  (Fig.  46),  this 
stem-like  character  is  less  evi- 
dent. The  potato  is  an  excel- 
lent example  of  the  short  and 
much-thickened  underground 
stem  known 
as  a  tuber. 
FIG.  48.  Bulb  of  hyacinth  Jt  may  be 

seen  from 

Fig.  47  that 

the  potatoes  are  none  of  them  borne  on  true 
roots,  but  only  on  subterranean  branches, 
which  are  stouter  and  more  cylindrical  than 
most  of  the  roots.  The  "  eyes  "  of  the  potato 
are  rudimentary  leaves  and  buds. 

Bulbs,  whether  coated  like  those  of  the 
onion  or  the  hyacinth  (Fig.  48),  or  scaly  like 
those  of  the  lily,  are  merely  very  short  and 
stout  underground  stems,  covered  with  closely 
crowded  scales  or  layers  which  represent  FIG.  49.  Longitudi- 

leaves  or  the  bases  of  leaves  (Fig.  49).  na!  sef  i(f  of  an 

onion  leaf 
The    variously   modified    forms   of   under- 

.  sea,  thickened  base  of 

ground  stems  just  discussed  illustrate  in  a     ieaf ,  forming  a  bulb 
marked  way  the  storage  of  nourishment  during     scale ;  s> thin  sheath 

J  fe       of  leaf;  bl,  blade  of 

the  winter,  or  the  rainless  season,  as  the  case  the  leaf  -int,  hollow 
may  be,  to  provide  the  material  for  rapid 
growth  during  the  active  season.  It  is  inter- 
esting to  notice  that  a  majority  of  the  early  flowering  herbs 
in  temperate  climates,  like  the  crocus,  the  snowdrop,  the  spring 


sea 


54 


STEMS 


beauty,  the  tulip,  and  the  skunk  cabbage,  owe  their  early  bloom- 
ing habit  to  richly  stored  underground  stems  of  some  kind,  or 
to  thick  fleshy  roots.  Many  of  these  very  early  blooming  plants 
are  woodland  species  which  must  hurry  through  most  of  the 
season's  growth  and  begin  to  mature  seed  before  the  shade  of 
the  trees  above  them  cuts  off  most  of  the  necessary  supply 
of  light  and  before  the  drought  of  summer  begins. 

67.  Condensed  stems.    The  plants  of  desert  regions  require, 
above  all,  protection  from  the  extreme  dryness  of  the  surround- 


FIG.  50.    A  globular  cactus 

ing  air,  and  usually  from  the  excessive  heat  of  the  sun.  Ac- 
cordingly, many  desert  plants  are  found  quite  destitute  of  ordi- 
nary foliage,  exposing  to  the  air  only  a  small  surface.  In  the 
globular  cactuses  (Fig.  50)  the  stem  appears  reduced  to  the 
shape  in  which  the  least  possible  surface  is  presented  by  a  plant 
of  given  bulk,  -  -  that  is,  in  a  somewhat  spherical  form.  Other 


LEAF-LIKE  STEMS 


55 


cactuses  are  cylindrical  or  prismatic,  while  still  others  consist 
of  flattened  joints ;  but  all  agree  in  offering  much  less  area  to 
the  sun  and  air  than  is  exposed  by  an  ordinary  leafy  plant. 

68.  Leaf-like  stems.  The  flattened  stems  of  some  kinds  of 
cactus,  especially  the  common  showy  Phyllocactus,  are  suffi- 
ciently like  fleshy  leaves,  with  their  dark  green  color  and  imita- 
tion of  a  midrib,  to  pass  for  leaves.  There  are,  however,  a  good 
many  cases  in  which  the  stern  takes  on  a  more  strikingly  leaf- 
like  form.  The  common  asparagus  sends  up  in  spring  shoots 
that  bear  large  scales  which  are  really  reduced  leaves.  Later  in 


FIG.  61.   A  spray  of  a  common  asparagus  (not  the  edible  species) 

the  season,  what  seem  like  thread-like  leaves  cover  the  much- 
branched  mature  plant,  but  these  green  threads  are  actually 
minute  branches,  which  perform  the  work  of  leaves  (Fig.  51). 
The  familiar  greenhouse  climber,  wrongly  known  as  smilax, 
properly  called  Myrsipliyllum,  bears  a  profusion  of  what  ap- 
pear to  be  delicate  green  leaves  (Fig.  52).  Close  study,  how- 
ever, shows  that  these  are  really  short  flattened  branches,  and 
that  each  little  branch  springs  from  the  axil  of  a  true  leaf,  I,  in 
the  form  of  a  minute  scale.  Sometimes  a  flower  and  a  leaf-like 
branch  spring  from  the  axil  of  the  same  scale. 


56 


STEMS 


Branches  which,  like  those  of  Myrsiphyllum,  so  closely  re- 
semble leaves  as  to  be  almost  indistinguishable  from  them  are 
called  dadopliylls,  meaning  branch  leaves. 

69.  The  range  of  modification  of  the  stem.  The  stem  may 
reach  a  length  of  many  hundred  feet,  as  in  the  tallest  trees,  in 
the  great  lianas  of  South  American  forests,  or  in  the  rattan  of 
Indian  jungles.  On  the  other  hand,  in  such  plants  as  the  prim- 
rose and  the  dandelion  the  stem  may  be  reduced  to  a  fraction 


FIG.  52.    Stem  of  Myrsiphyllum 

I,  scale-like  leaves;  cl,  cladophyll,  or  leaf-like  branch,  growing  in  the  axil  of  the 
leaf;  ped,  flower  stalk,  growing  in  the  axil  of  a  leaf 

of  an  inch  in  length.  It  may  take  on  apparently  root-like  forms, 
as  in  many  grasses  and  sedges,  or  become  thickened  by  under- 
ground storage  of  starch  and  other  plant  food,  as  in  the  iris,  the 
potato,  and  the  crocus.  Condensed  forms  of  stem  may  exist 
above  ground,  or,  on  the  other  hand,  branches  may  be  flat  and 
thin  enough  closely  to  imitate  leaves.  In  short,  the  stem  mani- 
fests great  readiness  in  adapting  itself  to  the  most  varied  con- 
ditions of  existence. 


CHAPTER  VII 


STRUCTURE  OF  THE  STEM 
STEM  OF  MONOCOTYLEDONOUS  PLANTS 

70.  External  characters.    The  most  familiar  of  the  larger 
monocotyledonous  plants  are  the  grass-like  ones,  such  as  Indian 
corn,  broom  corn,  and  bamboo,  the  green  briers  (Smilax),  and  the 
palms.     The   stem 

of  Indian  corn  con- 
sists of  a  series  o^ 
smooth,  slightly 
tapering  internodes 
connected  by  en- 
larged nodes.  Palm 
stems  often  have  a 
very  uneven  sur- 
face, due  to  the 
projecting  remains 
of  old  leafstalks 
(Fig.  53). 

71.  Internal 
structure.    A 
cross  section   of  a 
corn    stem    shows 
it  to  be  composed 
of    a    hard,    flinty 
rind,    inclosing    a 

very  soft  pith,  which  is  traversed  lengthwise  by  many  slender 
fibers  (Fig.  54).  The  fibers  are  arranged  in  a  somewhat  definite 
way,  the  smaller  ones  thickly  clustered  near  the  rind,  the  larger 
ones,  less  abundant,  toward  the  center. 

57 


FIG.  53.    Group  of  date  palms 


58 


STRUCTURE  OF  THE  STEM 


FIG.  54.   Diagrammatic  cross  section 
of  stem  of  Indian  corn 

cv,  fibro-vascular  bundles;  gc,  pithy 
material  between  bundles.  —  After 
Strasburger 


In  the  bamboo,  as  in  the  cane 
of  our  southern  canebrakes,  the 
interior  is  hollow,  with  a  hard, 
transverse  partition  at  each 
node. 

The  fibers  which  traverse  the 
pith  of  the  com  stem  are  not 
solid  cylinders,  but  are  built 
up  of  cells  of  several  kinds, 
around  and  between  tubes, 
somewhat  like  those  of  Fig.  62. 
The  whole  structure  is  known 
as  a  fibro-vascular  bundle;  that 
is,  a  bundle  of  fibers  and  ves- 
sels, or  tubes.  In  wroody  stems, 
such  as  those  of  the  bamboo 
or  palm,  the  bundles  are  closer 

together  and  much  harder  than  in  the  corn  stem.    The  outer 

rind  of  the  latter  is  composed  of  long,  thick-walled,  slender 

cells,  containing  much  silica  and  known  as  sclerencliyma  fibers. 
72.  Mechanical  function  of  the  manner 

of  distribution  of  material  in  monocoty- 

ledonous  stems.    The  well-known  strength 

and  lightness  of  the  straw  of  our  smaller 

grains  and  of  rods  of  cane  or  bamboo  are 

due  to  their  form.    It  can  readily  be  shown 

by  experiment  that  an  iron  or  steel  tube  of 

moderate  thickness,  like  a  piece  of  gas  pipe 

or  of  bicycle  tubing,  is  much  stiff er  than  a 

solid  rod  of  the  same  weight  per  foot.    The 

oat  straw,  the  stems  of  bulrushes,  the  cane 

of  our  southern  canebrakes,  and  the  bam- 
boo are  hollow  cylinders ;  the  cornstalk  is 

a  solid  cylinder,  but  filled  with  a  very  light  pith.    The  flinty  outer 

layer  of  the  stalk,  together  with  the  closely  packed  sclerencliyma 


FIG.  55.  Diagrammatic 
cross  section  of  stem 
of  bulrush  (Scirpus),  a 
hollow  cylinder  with 
strengthening  fibers 
After  Kerner 


THE  DICOTYLEDONOUS  STEM  59 

fibers  of  the  outer  rind  and  the  frequent  fibre-vascular  bundles 
just  within  this,  are  arranged  in  the  best  way  to  secure  stiffness. 
In  a  general  way,  then,  we  may  say  that  the  pith,  the  bundles, 
and  the  sclereiichymatous  rind  are  what  they  are  and  where 
they  are  to  serve  important  mechanical  purposes.  But  they  have 
other  uses  fully  as  important  (see  Chapter  vin). 

73.  Growth,  of  monocotyledonous  stems  in  thickness.    In 
most  woody  monocotyledonous  stems,  for  a  reason  which  will 
be  explained  later  in  this  chapter,  the  increase  in  thickness  is 
strictly  limited.    Such  stems,  therefore,  as  in  many  palms  and 
in  rattans,  are  less  conical  and  more  cylindrical  than  the  trunks 
of  ordinary  trees,  and  are  also  more  slender  in  proportion  to 
their  height. 

STEM  OF  DICOTYLEDONOUS  PLANTS 

74.  External  characters.    It  is  not  easy  to  make  any  gen- 
eral statements  about  the  external  characters  of  dicotyledonous 
stems,  on  account  of  their  very  great  variety  of  form.    The  stu- 
dent in  his  examination  of  twigs  in  connection  with  Chapter  VI 
has  learned  a  little  about  the  appearance  of  a  few  woody  stems. 
In  general,  the  nodes  are  much  less  marked  than  in  stems  of 
corn,  bamboo,  and  other  grass-like  forms.    In  the  case  of  decid- 
uous-leaved dicotyledonous  plants,  the  scars  left  by  fallen  leaves 
are  characteristic,  quite  unlike  those  mentioned  in  Sec.  70. 

75.  Internal  structure.1    If  one  begins  his  study  of  the  struc- 
ture of  dicotyledonous  stems  with  the  one-year-old  stem  of  a 
woody  plant  or  with  the  stem  of  some  such  robust  annual  as 
hemp,  sunflower,  or  the  great  ragweed,  he  will  find  it  to  be  com- 
posed of  a  somewhat  cylindrical  pith,  surrounded  by  a  layer 
of  wood  usually  of  pretty  even  thickness,  which  is  in  its  turn 
surrounded  by  a  layer  of  bark  (Fig.  56).2 

1  For  an  account  of  the  structure  of  the  pine  stem,  see  Sec.  352. 

2  Of  course  these  layers  are  nearly  cylindrical  tubes,  filled  by  pith  or  by 
wood  and  pith  respectively.    They  are  not  of  perfectly  circular  cross  section, 
and  they  taper  somewhat. 


60 


STRUCTURE  OF   THE   STEM 


The  wood  cylinder  may  be  discontinuous,  that  is,  broken  up 
into  separate  fibre-vascular  bundles,  as  shown  in  Fig.  57 ;  but 
even  then  the  position  of  the  wood  between  an  inclosed  pith 


FIG.  56.   Diagrammatic  cross  section  of  an  annual  dicotyledonous  stem 

p,  pith;  fv,  woody  or  fibro-vascular  bundles;  e,  epidermis;  b,  bundles  of  hard- 
bast  fibers  of  the  bark.    Somewhat  magnified.  —  After  Frank 


e    b     c  p 

FIG.  57.   Diagrammatic  cross  section  of  one-year-old  Aristolochia  stem 

c,  region  of  epidermis;  b,  hard-bast  fibers;  o,  outer  or  bark  part  of  a  bundle;  w, 
inner  or  woody  part  of  bundle;  c,  cambium  layer;  p,  region  of  pith;  m,  a 
medullary  ray.  Considerably  magnified 

The  space  between  the  hard  bast  and  the  bundles  is  occupied  by  thin-walled, 
somewhat  cubical  cells  of  the  bark  1 

and  an  inclosing  bark  is  notably  different  from  the  way  in 
which  the  bundles  are  scattered  in  monocotyledonous  stems. 

1  In  this  and  the  following  figure  the  relative  prominence  of  the  cambium 
layer  is  a  good  deal  exaggerated. 


FIG.  58.   One  bundle  from  the  preceding  figure 

to,  wood  cells ;  d,  vessels.   The  other  letters  are  as  in  Fig.  57.    Many  sieve  cells 
occur  in  the  region  just  outside  of  the  cambium  of  the  bundle,    x  100 


FIG.  59.  Stem  of  box  elder  one  year  old 

A,  lengthwise  (radial)  section;  B,  cross  section;  e,  epidermis;  ck,  cork;  6,  hard 
bast ;  s,  sieve  cells ;  c,  cambium ;  w,  wood  cells ;  m,  medullary  rays ;  d,  vessels ; 
p,  pith.  Much  magnified 

ftl 


w 


FIG.  60.  Part  of  cross  section  of  stem  of  flax 

e,  epidermis;  6,  hard  bast;  s,  sieve  cells;  w,  wood.    Much  magnified.  —  After 

Tschirch 


FIG.  61 


FIG.  62 


FIG.  61.   A  group  of  hard-bast  fibers 
a,  cut-off  ends ;  6,  lengthwise  section  of  fibers.  Greatly  magnified.  —After  Tschirch 

FIG.  62.   A  lengthwise  section  of  a  group  of  spiral  vessels  from  the  stem 

of  sunflower 

At  the  top  of  the  figure  some  of  the  spiral  threads  which  line  the  vessels  are  seen 

partly  uncoiled.    Greatly  magnified.  —  After  Frank 

62 


MATERIAL  FOR  STRENGTHENING  PURPOSES 


63 


76.  Disposition  of  material  for  strengthening  purposes.  Only 
two  of  the  many  ways  in  which  the  stem  is  strengthened  need 
be  mentioned  here.  In  a  majority  of  cases  it  owes  its  stiffness 
mainly  to  the  wood,  as  shown  in  Fig.  56.  But  not  infrequently 


FIG.  63  FIG.  64 

FIG.  63.   Part  of  a  sieve  tube  from  linden 
s,  sieve  plates  on  the  cell  wall,    x  about  900.  —  After  Thome 

FIG.  64.    Parts  of  sieve  tubes  as  found  in  plants  of  the  gourd  family 

ss,  a  sieve  plate  seen  edgewise ;  above  it  a  similar  one,  surface  view.    Greatly 
magnified.  —  After  Thome 

most  of  the  stiffening  material  consists  of  the  hard-bast  fibers 
found  in  the  bark.  It  is  this  layer  in  flax  (Fig.  60)  which  is 
utilized  in  the  manufacture  of  linen  thread  and  linen  fabrics. 


64  STRUCTURE   OF  THE  STEM 

77.  Structural  units  of  the  dicotyledonous  stem.    The  stu- 
dent should  already,  from  his  own  examinations,  have  learned 
a  good  deal  about  the  kinds  of  cells  and  cell  aggregates  which 
compose  the  stem.   The  preceding  figures  (Figs.  56-60)  will  serve 
to  illustrate  the  most  important  of  these,  and  Figs.  61-64  show 
some  of  them  more  in  detail. 

78.  Parenchyma,  prosenchyma,  and  collenchyma.    A  mass 
of  similar  cooperating  cells  is  called  a  tissue.1    Two  of  the  prin- 
cipal classes  not  previously  mentioned  which  occur  in  the  stem 
are  paremhymatous  tissue  and  prosencliymatous  tissue.    Paren- 
chyma is  well  illustrated  by  the  green  layer  of  the  bark,  by 
wood  parenchyma,  and  by  pith.    Its  cells  are  usually  somewhat 
roundish  or  cubical,  at  any  rate  not  many  times  longer  than 
wide,  and  at  first  rather  full  of  protoplasm.    Their  walls  are 
not   generally   very   thick.    Prosenchyma,  illustrated    by  hard 

bast  and  masses  of  wood  cells,  consists  of 
thick-walled  cells  many  times  longer  than 
wide,  containing  little  protoplasm  and  often 
having  little  or  no  cell  cavity. 

As  a  rule  the  stems  of  the  most  highly 
developed  plants  owe  their  toughness  and 
their  stiffness  mainly  to  prosenchymatous 
tissue.     In   some   stems,   particularly  the 
fleshy  ones,  the  stiffness  is,  however,  largely 
FIG.  65.    Collenchyma-   due  to  collenchyma,  a  kind  of  parenchyma 
tous  and  other  tissue   in  which  the  cellg  are  thickened  or  reen- 
from  stem  of  balsam  .  .  _r 

(Impatient)  forced  at  their  angles,  as  shown  in  Fig.  65. 

e,  epidermis;   c,  coiien-       79.  The  early  history  of  the  stem.   In 
chyma;  i,  intercellular   ^he  earliest  stages  of  the  growth  of  the 

spaces    between    large  •        .  .  .      .  .  ,,    , 

parenchyma  cells.—   stem   it    consists   entirely  of   thin-walled 
After  strasburger  an(j  rapidly  dividing  cells.    Soon,  however, 

the  various  kinds  of  tissue  which  are  found  in  the  full-grown 

stem  begin  to  appear. 

1  See  Strasburger,  Noll,  Schenck,  and  Schimper,  Text-Book  of  Botany, 
pp.  92-95,  2d  ed.,  London,  1903. 


SECONDARY  GROWTH 


65 


B 


In  Fig.  66  the  process  is  shown  as  it  occurs  in  the  castor 
bean.  At  ra,  in  B,  is  the  central  column  of  pith  surrounded 
by  eight  fibre-vascular  bundles,  fvy  each  of  which  contains  a 
number  of  vessels  arranged  in  a  somewhat  regular  manner  and 
surrounded  by  the  forerunners  of  the 
true  wood  cells. 

In  C  the  section  shows  a  consider- 
able advance  in  growth :  the  nbro- 
vascular  bundles  are  larger  and  are 
now  connected  by  a  rapidly  growing 
layer  of  tissue  c. 

As  growth  continues,  this  layer 
becomes  the  cambium  layer,  com- 
posed of  thin-walled  and  rapidly 
dividing  cells,  as  shown  in  Figs. 
67  and  68. 

80.  Secondary  growth.  From  the 
inside  of  the  cambium  layer  the  wood 
cells  and  ducts  of  the  mature  stem 
are  produced,  while  from  its  outer 
circumference  the  new  layers  of  the 
bark  proceed.  From  this  mode  of  in- 

crease    the    Stems    of    dicotyledonous 

plants  are  called   exogenous,  that  is, 

outside  growing.    The  presence  of  the 

cambium  layer  on  the  outside  of  the 

wood  in  early  spring  is  a  fact  well 

known  to  the  schoolboy  who  pounds 

the  cylinder  cut  from  an  alder,  willow, 

or  hickory  branch  until  the  bark  will 

slip  off  and  so  enable  him  to  make  a  whistle.    The  sweet  taste 

of  this  pulpy  layer,  as  found  in  the  white  pine,  the  slippery  elm, 

and  the  basswood,  is  a  familiar  evidence  of  the  nourishment 

which  the  cambium  layer  contains.     It  is  also,  as  might  be 

supposed,  very  rich  in  proteids. 


FIG.  66.  Transverse  section 
through  the  hypocotyl  of 
the  castor-oil  plant  at  vari- 
ous stages 

A,  after  the  root  has  just  ap- 
peared outside  the  testa  of  the 
seed ;  J3,  after  the  hypocotyl 
is  nearly  an  inch  long;  C,  at 
the  end  of  germination ;  r, 
cortex  (undeveloped  bark) ; 
m,  pith ;  st,  medullary  rays ; 
fv,  h'bro-vascular  bundles ; 
c,  layer  of  tissue  which  is  to 
develop  into  cambium.  Con- 
siderably magnified.  —  After 
Sachs 


66 


STRUCTURE  OF  THE  STEM 


With  the  increase  of  the  fibro-vascular  bundles  of  the  wood, 
the  space  between  them,  at  first  large,  becomes  less,  and  the 
pith,  which  extended  freely  out  toward  the  bark,  becomes  com- 
pressed into  thin  plates  so  as  to  form  medullary  rays. 

These  are,  as  already  stated,  of  value  in  storing  the  food  which 
the  plant  in  cold  and  temperate  climates  lays  up  in  the  sum- 
mer and  fall  for  use  in  the 
following  spring,  and  in 
the  very  young  stem  they 
serve  as  an  important 
channel  for  the  transfer- 
ence of  fluids  across  the 
stem  from  bark  to  pith,  or 
in  the  reverse  direction. 
On  account,  perhaps,  of 
their  importance  to  the 
plants,  the  cells  of  the 
medullary  rays  are  among 
the  longest  lived  of  all 
vegetable  cells,  retaining 
their  vitality  in  the  beech 
tree  sometimes,  it  is  said, 
for  more  than  a  hundred 
years. 

After  the  interspaces  be- 
tween the  first  fibro-vascu- 


FIG.  67.  Cross  section  of  a  three-year-old 
linden  twig 

e,  epidermis  and  corky  layer  of  the  bark; 
b,  bast ;  c,  cambium  layer  ;  r,  annual 
rings  of  wood.  Much  magnified.  —  After 


Kny 


lar  bundles   have   become 


filled  up  with  wood,  the  subsequent  growth  must  take  place  in 
the  manner  shown  in  Fig.  68.  The  cambium  of  the  original 
wedges  of  wood  fc,  and  the  cambium  ic  formed  between  these 
wedges,  continues  to  grow  from  the  inner  and  from  the  outer 
surface,  and  thus  causes  a  permanent  increase  in  the  diameter 
of  the  stem  and  a  thickening  of  the  bark,  which,  however,  usu- 
ally soon  begins  to  peel  off  from  the  outside  and  thus  remains 
pretty  constant  in  thickness. 


STEM  STRUCTURE  OF  CLIMBING  SHRUBS  67 

It  is  the  lack  of  any  such  ring  of  cambium  as  is  found  in 
dicotyledonous  plants,  or  even  of  permanent  cambium  in  the 
separate  bundles,  that  makes  it  impossible  for  the  trunks  of 
most  palm  trees  to  grow  indefinitely  in  thickness,  like  that  of 
an  oak  or  an  elm. 

81.  Stem  structure  of  climbing  shrubs.  Some  of  the  most 
remarkable  kinds  of  dicotyledonous  stems  are  found  in  climbing 
shrubs.  The  bundles,  as  shown  in  Fig.  57,  are  much  more  dis- 
tinct than  in  most  other  woody  stems.  It  is  evident  that  this 


Jc 
FIG.  68.   Diagram  to  illustrate  secondary  growth  in  a  dicotyledonous  stem 

R,  the  first-formed  bark ;  p,  mass  of  sieve  cells ;  ifp,  mass  of  sieve  cells  between 
the  original  wedges  of  wood ;  /c,  cambium  of  wedges  of  wood ;  ic,  cambium 
between  wedges ;  b,  groups  of  bast  cells ;  //?,  wood  of  the  original  wedges ;  ifh, 
wood  formed  between  wedges;  x,  earliest  wood  formed ;  M,  pith.  —  After  Sachs 

is  for  the  sake  of  leaving  the  stem  flexible  for  twining  purposes, 
just  as  a  wire  cable  is  adapted  to  be  wound  about  posts  or  other 
supports,  while  a  solid  steel  or  iron  rod  of  the  same  size  would 
be  too  stiff  for  this  use. 

82.  Interruption  of  annual  rings  by  branches  ;  knots.  When 
a  leaf  bud  is  formed  on  the  trunk  or  branch  of  a  dicotyledonous 
tree  it  is  connected  with  the  wood  by  fibro-vascular  bundles. 


68 


STRUCTURE   OF   THE  STEM 


As  the  bud  develops  into  a  branch,  the  few  bundles  which  it 
originally  possessed  increase  greatly  in  number,  and  at  length, 
as  the  branch  grows,  form  a  cylinder  of 
wood  which  cuts  across  the  annual  rings, 
as  shown  in  Fig.  69.  This  interruption  to 
the  rings  is  a  knot,  such  as  one  often  sees 
in  boards  and  planks.  If  the  branch  dies 
long  before  the  tree  does,  the  knot  may 
be  buried  under  many  rings  of  wood.  What 
is  known  as  "clear"  lum- 
ber is  obtained  from  trees 
that  have  grown  in  a 
dense  forest,  so  that  the 
lower  branches  of  the 
larger  trees  were  killed 
by  the  shade  many  years 
before  the  tree  was  felled. 
In  pruning  fruit  trees 
a  knot  in  a  tree  trunk  or  shade  trees  the 

R,  cut-off  end  of  stick,  branches  which    are   re- 
showing  annual  rings ;  ^      ^         •,  ^    -. 

K,  knot  formed  by  m0ved    should    be    Cut 

growth  of  a  branch.  —  close    to    the    trunk.     If 

After  Roth  A.  .     .       , 

this  is  done,  the  growth 
of  the  trunk  will  bury  the  scar  before  decay 
sets  in. 

83.  Grafting.  When  the  cambium  layer 
of  any  vigorously  growing  stem  is  brought  in  FIG.  70.  Grafting 
contact  with  the  same  layer  in  another  stem  At  the  left  scion  and 
of  the  same  kind  or  a  closely  similar  kind  of 
plant,  the  two  may  grow  together  to  form  a 
single  stem  or  branch.  This  process  is  called 
grafting,  and  is  much  resorted  to  in  order  to 
secure  apples,  pears,  etc.,  of  any  desired  kind 
(Fig.  70).  A  twig  known  as  the  scion  from  a  plant  of  the  chosen 
variety  may  be  grafted  upon  another  individual  of  similar  kind 


FIG. 


Formation    of 


stock  are  shown 
ready  to  be  united ; 
at  the  right  they  are 
joined  and  ready  to 
cover  with  grafting 
wax.  —  After  Perci- 
val 


GRAFTING 


69 


known  as  the  stock,  and  the  resulting  stems  will  bear  the 
wished-for  sort  of  fruit.  Often  one  species  is  grafted  on  another, 
as  the  pear  on  the  quince  or  the  apple.  Karely  trees  differing 
as  much  as  the  chestnut  and  the  oak  may  be  grafted  together. 


FIG.  71.  Two  ash  trees  naturally  grafted  together 
After  Werthner 

Sometimes  grafting  comes  about  naturally  by  the  branches  of  a 
tree  chafing  against  one  another  until  the  bark  is  worn  away 
and  the  cambium  layer  of  each  is  in  contact  with  that  of  the 
other,  or  two  separate  trees  may  be  joined  by  natural  grafting 
into  a  nearly  cylindrical  double  trunk,  as  is  shown  in  Fig.  71. 


70 


STRUCTURE   OF  THE  STEM 


84.  Comparison  of  the  monocotyledonous  and  the  dicotyledo- 
nous stem.1 


General 
Structure  : 


Structure  of 
Bundles  : 


MONOCOTYLEDONOUS 
STEM 

A  hard  rind  of  rather 
uniform  structure. 
Bundles  intermixed 
with  the  pith. 


Bundles  dosed;  that 
is,  without  permanent 
cambium. 


DICOTYLEDONOUS 
STEM 

A  complex  bark ,  usually 
on  young  shoots,  consist- 
ing of  an  epidermis,  a 
corky  layer,  a  green  layer, 
and  a  layer  of  bast.  A 
layer  of  cambium.  Wood 
in  annual  rings.  Pith  in 
a  cylinder  at  the  center. 

Bundles  open,  with  per- 
manent cambium. 


Growth  in 
Thickness  : 


Cells  of  mature  parts 
of  stem  expand  some- 
what, but  (in  most 
palms)  new  ones  are 
not  formed. 


New  wood  cells  formed 
throughout  growing  sea- 
son from  cambium  ring. 


1  This  comparison  applies  only  to  most  of  the  woody  or  tree-like  stems. 


CHAPTER  VIII 
LIVING  PARTS  OF  THE  STEM;    WORK  OF  THE  STEM 

85.  Active  portions  of  the  stems  of  trees  and  shrubs.    In 

annual  plants  generally,  and  in  the  very  young  shoots  of  shrubs 
and  trees,  there  are  stomata  (singular  stoma,  meaning  mouth), 
or  breathing  pores,  which  occur  abundantly  in  the  epidermis, 
serving  for  the  admission  of  air  and  the  escape  of  moisture, 
while  the  green  layer  of  the  bark  answers  the  same  purpose 
that  is  served  by  the  green  pulp  of  the  leaf  (Chapter  xn).  For 
years,  too,  the  spongy  lenticels,  which  succeed  the  stomata  and 
occur  scattered  over  the  external  surface  of  the  bark  of  trees 
and  shrubs,  serve  to  admit  air  to  the  interior  of  the  stem.  The 
lenticels  at  first  appear  as  roundish  spots,  of  very  small  size;  but 
as  the  twig  or  shoot  on  which  they  occur  increases  in  diameter, 
the  lenticel  becomes  spread  out  at  right  angles  to  the  length  of 
the  stem,  so  that  it  sometimes  becomes  a  longer  transverse  slit 
or  scar  on  the  bark,  as  in  the  cherry  and  the  birch  and  the 
elder.  But  in  the  trunk  of  a  large  tree  often  no  part  of  the 
bark  except  the  inner  layer  is  alive.  The  older  portions  of 
the  bark,  such  as  the  highly  developed  cork  of  the  cork  oak, 
sometimes  cling  for  years  after  they  are  dead  and  useless  ex- 
cept as  a  protection  for  the  parts  beneath  against  mechanical 
injuries  or  against  cold.  But  in  many  cases,  as  in  the  shellbark 
hickory  and  the  grapevine,  the  old  bark  soon  falls  off  in  strips ; 
or  as  in  birches  it  finally  peels  off  in  bands  around  the  stern. 

The  cambium  layer  is  very  much  alive,  and  so  is  the  young 
outer  portion  of  the  wood.  Testing  this  sapwood,  particularly 
in  winter,  shows  that  it  is  rich  in  starch  and  proteids. 

The  heartwood  of  a  full-grown  tree  is  hardly  living,  unless 
the  cells  of  the  medullary  rays  retain  their  vitality,  and  so  it  is 

71 


72  WORK  OF  THE  STEM 

probable  that  wood  of  this  kind  is  chiefly  useful  to  the  tree  by 
giving  stiffness  to  the  trunk  and  larger  branches,  thus  preventing 
them  from  being  easily  broken  by  storms. 

It  is  therefore  possible  for  a  tree  to  flourish,  sometimes  for 
centuries,  after  the  heartwood  has  much  of  it  rotted  away  and 
left  the  interior  of  the  trunk  hollow,  as  shown  in  Fig.  72. 

86.  Uses  of  the  components  of  the  stem.  There  is  a  marked 
division  of  labor  among  the  various  groups  of  cells  that  make 
up  the  stem  of  ordinary  dicotyledons,  particularly  in  the  stems 
of  trees,  and  it  will  be  best  to  explain  the  uses  of  the  kinds  of 
cells  as  found  in  trees  rather  than  in  herbaceous  plants.  A  few 
of  the  ascertained  uses  of  the  various  tissues  are  these : 

The  pith  forms  a  large  part  of  the  bulk  of  very  young  shoots, 
since  it  is  a  part  of  the  tissue  of  comparatively  simple  structure 
amid  which  the  fibre-vascular  bundles  arise.  In  mature  stems 
it  becomes  rather  unimportant,  though  it  often  continues  for  a 
long  time  to  act  as  a  storehouse  of  food. 

The  medullary  rays  in  the  young  shoot  serve  as  a  channel 
for  the  transference  of  water  and  plant  food  in  a  liquid  form 
across  the  stem,  and  they  often  contain  much  stored  food. 

The  vessels  carry  water  upward  through  the  stem  in  certain 
plants. 

The  wood  cells  of  the  heartwood  are  useful  only  to  give  stiff- 
ness to  the  stem.  Those  of  the  sapwood,  in  addition  to  this 
work,  have  to  carry  most  of  the  water  from  the  roots  to  the 
leaves  and  other  distant  portions  of  the  plant. 

The  cambium  layer  is  the  region  in  which  the  annual  growth 
of  the  tree  takes  place. 

Sieve  tubes  form  the  most  important  portion  of  the  inner  bark, 
carrying  elaborated  plant  food  from  the  leaves  toward  the  roots. 

The  green  layer  of  the  bark  in  young  shoots  does  much  toward 
collecting  nutrient  substances,  or  raw  materials,  and  preparing 
the  food  of  the  plant  from  air  and  water,  but  this  work  may 
be  best  explained  in  connection  with  the  study  of  the  leaf 
(Chapter  xn). 


MOVEMENT  OF  WATER  IN  THE   STEM 


73 


FIG.  72.    Pioneer's  cabin,  a  hollow  giant  redwood  (Sequoia) 
After  White 

87.  Movement  of  water  in  the  stem.  The  student  has  already 
learned  that  large  quantities  of  water  are  taken  up  by  the  roots 
of  plants. 


74 


WORK  OF  THE  STEM 


Having  become  somewhat  acquainted  with  the  structure  of 
the  stem,  he  is  now  in  a  position  to  investigate  the  question  how 
the  various  fluids,  commonly  known  as  sap,  travel  about  in  it.1 

It  is  important  to  notice 
that  sap  is  by  no  means 
the  same  substance  every- 
where and  at  all  times. 
As  it  first  makes  its  way 
by  osmotic  action  inward 
through  the  root  hairs  of 
the  growing  plant  it  differs 
but  little  from  ordinary 
spring  water  or  well  water. 
The  liquid  which  flows 
from  the  cut  stem  of  a 
"  bleeding  "  tree  or  grape- 
vine  which  has  been 
pruned  just  before  the  buds 
have  begun  to  burst  in  the 
spring  is  mainly  water, 
often  with  a  little  dissolved 
organic  acids,  proteids,  and 
sugar.  The  sap  which  is 

FIG.  73.    Channels  for  the  movement  of      °btained  f rom  maPle  tr66S 
water,  upward  and  downward  in     late     winter     or     early 

The  heavy  black  lines  in  roots,  stems,  and  Spring,  and  is  boiled  down 
leaves  show  the  course  of  the  fibro-vascular  £or  gjrup  or  Sugar   is  richer 
bundles  through  which  the  principal  move- 
ments of  water  take  place.  —  After  Frank  ill  nutritious  material  than 
and  Tschirch  j_v 


while  the  elaborated  sap  which  is  sent  so  abundantly  into  the  ear 
of  corn  at  its  period  of  filling  out,  or  into  the  growing  pods  of 
beans  and  peas,  or  into  the  rapidly  forming  acorn  or  the  chestnut, 
contains  great  stores  of  food  suited  to  sustain  plant  or  animal  life. 


1  See  the  paper  on  "The  So-called  Sap  of  Trees  and  its  Movements,"  by 
Professor  Charles  R.  Barnes,  Science,  Vol.  XXI,  p.  535. 


MOVEMENT  OF  LIQUIDS  IN  THE  STEM 


75 


From  the  familiar  facts  that  ordinary  forest  trees  apparently 
flourish  as  well  after  the  almost  complete  decay  and  removal  of 
their  heartwood,  and  that  many  kinds  will  live  and  grow  for  a 
considerable  time  after  a  ring  of  bark  extending  all  round  the 
trunk  has  been  removed,  it  may  readily  be  inferred  that  the 
crude  sap  in  trees  must  rise  through  some  portion  of  the  newer 
layers  of  the  wood.  A  tree  girdled  by  the  removal  of  a  ring  of 


FIG.  74.  A  cutting  girdled  and 
sending  down  roots  from  the 
upper  edge  of  the  girdled 
ring 


FIG.  75.  Diagrammatic  cross  section  of  a 
bundle  from  sugar  cane,  showing  chan- 
nels for  water  and  dissolved  plant  food 

Water  travels  upward  through  the  vessels  d 
and  through  the  wood  cells  in  the  region 
marked  w.  Water  with  dissolved  plant 
food  travels  downward  through  the  sieve 
tubes  in  the  region  s.  Magnified 


sapwood  promptly  dies.  After  the  removal  of  a  ring  of  bark  a 
tree  dies  from  starvation  of  the  roots  (Sec.  88  ;  also  see  Fig.  394). 
88.  Downward  movement  of  liquids.  Most  dicotyledonous 
stems,  when  stripped  of  a  ring  of  bark  and  then  set  in  water, 
as  shown  in  Fig.  74,  and  covered  with  a  bell  jar,  develop  roots 
only  at  or  near  the  upper  edge  of  the  stripped  portion.  This 
would  seem  to  prove  that  such  stems  send  their  building  mate- 
rial —  the  elaborated  sap  —  largely,  at  any  rate,  down  through 


76  WORK  OF   THE  STEM 

the  bark.  Its  course  is  undoubtedly  for  the  most  part  through 
the  sieve  tubes  (Figs.  63,  64),  which  are  admirably  adapted  to 
convey  liquids.  In  addition  to  these  general  upward  and  down- 
ward movements  of  sap,  there  must  be  local  transfers  laterally 
through  the  stem,  and  these  are  at  times  of  much  importance 
to  the  plant. 

Since  the  liquid  building  material  travels  straight  down  the 
stem,  that  side  of  the  stem  on  which  the  manufacture  of  such 

material  is  going  on  most  rapidly 
should  grow  fastest.  Plant  food  is 
made  out  of  the  raw  materials  by  the 
leaves,  and  so  the  more  leafy  side  of  a 
tree  forms  thicker  rings  than  the  less 
leafy  side,  as  shown  in  Fig.  76. 

89.  Rate  of  movement  of  water  in 
the  stem.    There  are  many  practical 
FIG.  76.   Unequal  growth  of    difficulties  in  the  way  of  ascertaining 
rings  of  wood  in  a  nearly   exactly  how  fast  the  watery  sap  travels 
horizontal  stem  of  juniper    from  the  root  to  the  leaveg     jfc  ^  how_ 

Natural  size  ever^  eagv  to  ^ justrate  experimentally 

the  fact  that  it  does  rise,  and  to  give  an  approximate  idea  of 
the  time  required  for  its  ascent.  The  best  experiment  for  be- 
ginners is  one  which  deals  with  an  entire  plant  under  natural 
conditions ;  that  is,  by  allowing  a  plant  to  wilt  from  lack  of 
water,  then  watering  it  freely  and  noting  how  soon  the  leaves 
begin  to  recover  their  natural  appearance  and  positions. 

The  interval  of  time  will  give  a  very  rough  idea  of  the  time 
of  transfer  of  water  through  the  roots  and  the  stem  of  the  plant. 
From  this,  by  measuring  the  approximate  distance  traveled,  a 
calculation  could  be  made  of  the  number  of  inches  per  minute 
that  water  travels  in  this  particular  kind  of  plant,  through  a 
route  which  is  partly  roots,  partly  stem,  and  partly  petiole. 
Still  another  method  is  to  immerse  the  cut  ends  of  leafy  stems 
in  eosin  solution  and  note  carefully  the  rate  of  ascent  of  the 
coloring  liquid.  This  plan  is  likely  to  give  results  that  are  too 


CAUSES  OF  MOVEMENTS  OF  WATER  IN  THE  STEM     77 

low ;  still  it  is  of  some  use.  It  has  given  results  varying  from 
34  inches  per  hour  for  the  willow  to  880  inches  per  hour  for 
the  sunflower.  A  better  method  is  to  introduce  the  roots  of  the 
plant  which  is  being  experimented  upon  into  a  weak  solution 
of  some  chemical  substance  which  is  harmless  to  the  plant  and 
which  can  readily  be  detected  anywhere  in  the  tissues  of  the 
plant  by  chemical  tests.  Proper  tests  are  then  applied  to  por- 
tions of  the  stem  which  are  cut  from  the  plant  at  short  intervals 
of  time. 

Compounds  of  the  metal  lithium  are  well  adapted  for  use  in 
this  mode  of  experimentation,  if  a  spectroscope  is  available  to 
test  for  its  presence. 

90.  Causes  of  movements  of  water  in  the  stem.  Some  of  the 
phenomena  of  osmosis  were  explained  in  Sees.  48—51,  and  the 
work  of  the  root  hairs  was  described  as  due  to  osmotic  action. 

That  portion  of  the  sap  pressure  which  originates  in  the  roots 
(Sec.  38),  being  apparently  able  to  sustain  a  column  of  water 
only  80  or  90  feet  high  at  the  most  (and  usually  less  than  half 
this  amount),  would  be  quite  insufficient  to  raise  the  sap  to  the 
tops  of  the  tallest  trees,  since  many  kinds  grow  to  a  height 
of  more  than  100  feet.  Our  California  "big  trees,"  or  Sequoias, 
reach  the  height  of  over  300  feet,  and  an  Australian  species  of 
Eucalyptus,  it  is  said,  sometimes  towers  up  to  470  feet.  Eoot 
pressure,  then,  may  serve  to  start  the  soil  water  on  its  upward 
journey,  but  some  other  force  or  forces  must  step  in  to  carry  it 
the  rest  of  the  way.  What  these  other  forces  are  is  still  a  matter 
of  discussion  among  botanists. 

The  slower  inward  and  downward  movement  of  the  sap  may 
be  explained  as  due  to  osmosis.  For  instance,  in  the  case  of 
growing  wood  cells,  sugary  sap  descending  from  the  leaves  into 
the  stem  gives  up  part  of  its  sugar  to  form  the  cellulose  of 
which  the  wood  cells  are  being  made. 

This  loss  of  sugar  leaves  the  sap  rather  more  watery  than 
usual,  and  osmosis  carries  it  from  the  growing  wood  to  the 
leaves,  while  at  the  same  time  a  slow  transfer  of  the  dissolved 


78  WORK  OF  THE  STEM 

sugar  is  set  up  from  leaves  to  wood.  The  water  is  thrown  off 
in  the  form  of  vapor  as  fast  as  it  reaches  the  leaves,  so  that  they 
do  not  become  distended  with  water,  while  the  sugar  is  changed 
into  cellulose  and  built  into  new  wood  cells  as  fast  as  it  reaches 
the  region  where  such  cells  are  being  formed. 

Plants  in  general 1  readily  change  starch  to  sugar,  and  sugar 
to  starch.  When  they  are  depositing  starch  in  any  part  of  the 
root  or  stem  for  future  use,  the  withdrawal  of  sugar  from  those 
portions  of  the  sap  which  contain  it  most  abundantly  gives  rise 
to  a  slow  movement  of  dissolved  particles  of  sugar  in  the  direc- 
tion of  the  region  where  starch  is  being  laid  up. 

91.  Storage  of  food  in  the  stem.    The  reason  why  the  plant 
may  profit  by  laying  up  a  food  supply  somewhere  inside  its 
tissues  has  already  been  suggested  (Sec.  33). 

The  most  remarkable  instance  of  storage  of  food  in  the  stem 
is  probably  that  of  sago  palms,  which  contain  an  enormous 
amount,  sometimes  as  much  as  eight  hundred  pounds,  of  starchy 
material  in  a  single  trunk.  But  the  commoner  plants  of  tem- 
perate regions  furnish  abundant  examples  of  deposits  of  food 
in  the  stem. 

92.  Storage  in  underground  stems.    The  branches  and  trunk 
of  a  tree  furnish  the  most  convenient  place  in  which  to  deposit 
food  during  winter  to  begin  the  growth  of  the  following  spring. 
But  in  those  plants  which  die  down  to  the  ground  at  the  begin- 
ning of  winter  the  storage  must  be  either  in  the  roots  or  in 
underground  portions  of  the  stem. 

Kootstocks,  tubers,  and  bulbs  seem  to  have  been  developed 
by  plants  to  answer  as  storehouses  through  the  winter  (or  in 
some  countries  through  the  dry  season)  for  the  reserve  materials 
which  the  plant  has  accumulated  during  the  growing  season. 
The  commonest  tuber  is  the  potato,  and  this  fact  and  the  points 
of  interest  which  it  represents  make  it  especially  desirable  to 
use  for  a  study  of  the  underground  stein  in  a  form  most  highly 
specialized  for  the  storage  of  starch  and  other  valuable  products. 
1  Not  including  most  of  the  spore  plants. 


OCCURRENCE  OF  SUGAR  IN  THE  STEM  79 

It  is  evident  that  in  the  potato  we  have  to  do  with  a  very 
highly  modified  form  of  stem.  The  corky  layer  of  the  bark  is 
well  represented,  and  the  loose  cellular  layer  beneath  is  much 
developed ; '  wood  is  almost  lacking,  but  the  pith  is  greatly 
developed  and  constitutes  the  principal  bulk  of  the  tuber. 
All  this  is  readily  understood  if  we  consider  that  the  tuber, 
buried  in  and  supported  by  the  earth,  does  not  need  the  kinds 
of  tissue  which  give  strength,  but  only  those  which  are  well 
adapted  to  store  the  requisite  amount  of  food. 

93.  Occurrence  of  sugar  in  the  stem.  Grape  sugar  is  an 
important  substance  among  those  used  for  food  by  the  plant.  It 
received  its  name  from  the  fact  that  it  was  formerly  obtained 
for  chemical  examination  from  grapes.  Old  dry  raisins  usually 
show  little  masses  of  whitish  material  scattered  over  the  skin 
which  are  nearly  pure  grape  sugar.  Commercially  it  is  now 
manufactured  on  an  enormous  scale  from  starch  by  boiling  with 
diluted  sulphuric  acid.  In  the  plant  it  is  made  from  starch 
by  processes  as  yet  imperfectly  understood,  and  another  sugar, 
called  maltose,  is  made  from  starch  in  the  seed  during  germina- 
tion. Sugar  is  not  as  well  adapted  for  reserve  deposits  as  starch, 
since  it  ferments  easily  and  may  escape  by  osmosis  from  tissues 
which  contain  it.  In  the  onion  bulb  it  is  stored  in  considerable 
quantities  and  may  be  detected  by  a  simple  chemical  test. 


CHAPTER  IX 


BUDS 

94.  Structure  of  winter  buds.    Dissection  of  most  winter 
buds  shows  that  they  are  composed  of  an  outer  covering  of 

tough,  often  hairy  or  resin-covered 
scales  and  an  interior  mass  of  small 
undeveloped  leaves,  closely  packed 
together.  Not  infrequently  a  rudi- 
mentary flower  cluster  occupies  the 
central  portion  of  the  bud. 

95.  Nature  of  bud  scales.  The 
fact  that  the  bud  scales  are  in  cer- 
tain cases  merely  imperfectly  de- 
veloped leaves  or  leafstalks  is  often 
clearly  manifest  from  the  series  of 
steps  connecting  the  bud  scale  on 
the  one  hand  with  the  young  leaf 
on  the  other,  which  may  be  found 
in  many  opening  buds,  as  illus- 
trated by  Fig.  77.  In  other  buds 
the  scales  are  not  imperfect  leaves, 
but  the  little  appendages  (stipules, 
Figs.  89,  90)  which  occur  at  the 
bases  of  leaves.  This  kind  of  bud 

FIG.  77.   Dissected  bud  of  buck-  ^   1S    6SPecially   wel1  shown  in 

eye  (sEsculus    macrostachya),  the   magnolia    and   the   tulip  tree 

showing  transitions  from  bud  and  in  the  familiar  "  rubber  plant " 

scales  to  leaves  (picus  dastica) 

96.   Naked  buds.    All  the  kinds  above  mentioned  are  resting 
buds,  and  in  temperate  or  cold  climates  winter  luds,  capable  of 


SCALY  BUDS  AND  NAKED  BUDS 


81 


FIG.  79.   Alternate  leaves  of  cultivated  cherry,  with 
buds  in  their  axils,  in  October 

living    through   the   colder   months   of  the 
year,  and  are  also  scaly  buds. 

In  the  herbs  of  temperate  climates,  and 
even  in  shrubs  and  trees  of  tropical  regions, 
the  buds  are  often  naked;  that  is,  nearly  or 
quite  destitute  of  scaly  coverings  (Fig.  78). 
These  are  best  suited  for  a  season  or  a 
climate  which  is  both  warm  and  moist. 
The  scales,  of  whatever  sort,  with  their  coat- 
ings of  hair  or  of  resinous  material,  are  of 
use  mainly  in  protecting  buds  from  sudden 
changes  of  temperature  or  too  rapid  loss 
of  water.  The  latter,  in  climates  like  that 
of  southern  California  or  the  Mediterranean 
coast,  would  be  during  the  rainless  summer. 


FIG.  78 

Tip  of  branch  of 
Ailanthus  in  winter 
condition,  showing 
very  large  leaf  scars 
and  nearly  naked 
buds 


82 


BUDS 


In  most  cold  or  temperate  climes  it  would  be  during  the  winter, 
when  little  water  can  be  drawn  from  the  soil  (Sec.  39). 

97.  Position  of  buds.    The  distinction  between  lateral  and 
terminal  buds  has  already  been  alluded  to  (Sec.  57). 

The  plumule  is  the  first  terminal  bud 
which  the  plant  produces.  Lateral  buds 
are  usually  axillary,  as  shown  in  Fig.  79, 
that  is  they  grow  in  the  angle  formed 
by  the  leaf  with  the  stem  (Latin,  axilla, 
armpit) ;  but  not  infrequently  there  are 
several  buds  grouped  in  some  way  about 
a  single  leaf  axil,  either  one  above  the 
other,  as  in  the  butternut  (Fig.  80),  or 


FIG.  80.    Accessoiy 
buds  of  butternut 

I,  leaf  scar ;  ax,  axil- 
lary bud ;  a,  a',  ac- 
cessory  buds;  t, 
terminal  bud.  Re- 
duced. 


FIG.  81.   Accessory  buds  of  box  elder 
(Negundo) 

A,  front  view  of  group;  />,  two  groups 
seen  in  profile.     Magnified 


grouped  side  by  side,  as  in  the  red  maple,  the  cherry,  and  the 
box  elder  (Fig.  81). 

In  these  cases,  all  the  buds,  except  the  axillary  one,  are 
called  accessory  or  supernumerary  buds.    Those  which  appear  in 


LEAF  BUDS  AND  FLOWER  BUDS 


83 


irregular  positions,  as  on  roots,  on  unusual  parts  of  the  stem,  or 
on  leaves  (Fig.  88),  are  called  adventitious  buds. 

98.  Leaf  buds  and  flower  buds ;  the  bud  an  undeveloped 
branch.  Buds  are  of  three  principal  classes  :  leaf  buds,  in  which 
the  parts  inside  of  the  scales  develop  into  leaves,  and  their  cen- 
tral axes  into  stems  ;  mixed  buds,  which  contain  both  leaves  and 
flowers  in  an  undeveloped  condition ;  and  flower  buds,  which 
contain  the  rudiments  of  flowers  only. 

Sometimes,  as  in  the  black  walnut  and  the  butternut,  the 
leaf  buds  and  flower  buds  are  readily  distinguishable  by  their 


A  B 


FIG.  82 

A,  a  pear  leaf  bud  in  autumn;  7>,  a  leafy  shoot-derived  from  A,  as  seen  in  the 
middle  of  the  following  summer,  with  flower  bud  at  tip;  C,  the  fruit  spur,  B, 
in  autumn,  after  the  fall  of  the  leaves.  —  After  Percival 

difference  in  form ;  while  in  other  cases,  as  in  the  cultivated 
cherry,  the  difference  in  form  is  but  slight.  In  many  plants,  as 
the  lilac,  there  is  a  notable  difference  in  size. 

The  rings  of  scars  about  the  twig,  shown  in  Figs.  79  and  84, 
mark  the  place  where  the  bases  of  bud  scales  were  attached. 
A  little  examination  of  the  part  of  the  twig  which  lies  above 
this  ring,  as  shown  in  Fig.  79,  will  lead  one  to  the  conclu- 
sion that  this  portion  has  all  grown  in  the  one  spring  and  sum- 
mer since  the  bud  scales  of  that  particular  ring  dropped  off. 


84 


BUDS 


Following  out  this  suggestion,  it  is  easy  to  reckon  the  age  of 
any  moderately  old  portion  of  a  branch,  since  it  is  equal  to  the 


— '1905 


FIG.  83.    Fruit  bud  of  pear  (same  as  C,  of  Fig.  82),  showing  tits  development 

A,  opening  in  spring;  B,  later,  developing  flowers  and  leaves;  C,  later  still:  only 
one  flower  has  produced  a  fruit,  the  rest  having  fallen  off.  Below  it  is  a  lateral 
hud  which  will  continue  the  spur  next  year.  — After  Percival 

number  of  segments  between  the  rings.  In 
/  rapidly  growing  shoots  of  willow,  poplar,  and 
-1906  similar  trees,  five  or  ten  feet  may  be  the 
growth  of  a  single  year,  while  in  the  lateral 
twigs  of  the  hickory,  apple,  or  cherry,  the 
yearly  increase  may  be  but  a  fraction  of  an 
inch.  Such  "  spurs "  as  are  shown  in  Figs. 
82-84  are  of  little  use  in  the  permanent 
growth  of  the  tree,  and  poplars,  elms,  soft 
maples,  and  other  trees  shed  the  oldest  of 
these  every  year.  In  any  case  the  growth  is 
but  the  development  of  the  bud,  which  may  be 
F,G.  84.  A  slowly  grown  twig  of  regarded  as  an  undeveloped  stem 
cherry,  three  inches  long  and  or  branch,  with  its  internodes  so 
about  ten  years  old  shortened  that  successive  leaves 

The  pointed  hud  /  is  a  leaf  hud ;  the      seem  almost  to  spring  from  the 

more  obtuse  accessory  huds  /,  f 

are  flower  huds  Same  point. 


VERNATION 


85 


99.  Vernation.    The  arrangement  of  leaves  in  the  bud  is  called 
vernation;  some  of  the  principal  modes  are  shown  in  Fig.  86. 
In  the  cherry  the  two  halves  of  the  leaf  are  folded  together  flat, 
with  the  under  surfaces  outward ;  in  the  walnut  the 
separate  leaflets,  or  parts  of  the  leaf,  are  folded  flat 

and  then  grouped  into  a  sort  of  cone ;  in  the  snow- 
ball each  half  of  the  leaf  is  plaited  in  a  somewhat 
fan-like  manner,  and  the  edges  of  the  two  halves 
are  then  brought  round  so  as  to  meet ;  in  the  lady's 
mantle  the  fan-like  plaiting  is 
very  distinct ;  in  the  wood  sorrel 
each  leaflet  is  folded  smoothly, 
and  then    the    three  leaflets 
packed  closely  side  by  side.    All 
these  modes   of   vernation,  and  ax 
many  others,  often  characteristic 
of  groups  of  plants,  have  received 
descriptive  names  by  which  they 
are  known  to  botanists. 

100.  Importance  of  verna- 
tion.   The  significance  of  verna- 
tion is  best  understood  by 
considering  that  there  are  two 
important  purposes  to  be  served : 
the  leaves   must  be   stowed  as 
closely  as  possible  in  the  bud, 
and    upon    beginning    to    open 

they  must  be  protected  from  too  great  heat  and  dryness  until 
they  have  reached  a  certain  degree  of  firmness.  It  may  be 
inferred  from  Fig.  86  that  it  is  common  for  very  young  leaves 
to  stand  vertically.  This  protects  them  considerably  from  the 
scorching  effect  of  the  sun  at  the  hottest  part  of  the  day. 
Many  young  leaves,  as,  for  instance,  those  of  the  silver-leafed 
poplar,  the  pear,  the  beech,  and  the  mountain  ash,  are  sheltered 
and  protected  from  cold,  dryness,  and  the  attacks  of  small 


FIG.  85 

B,  a  twig  of  European  elm;  A,  a 
longitudinal  section  of  the  buds  of 
B  (considerably  magnified) ;  ax,  the 
axis  of  the  bud,  which  will  elongate 
into  a  shoot ;  sc,  leaf  scars.  —  After 
Behrens 


86 


BUDS 


insects  by  a  coating  of  wool  or  down,  which  they  afterwards 
lose.    The  leaves  of  the  tulip  tree  are  inclosed  for  a  little  time 


FIG.  86.  Types  of  vernation 

1,  2,  cherry;  3,  4,  European  walnut;  5,  6,  snowball;  7,  lady's  mantle;  8,  oxalis. 

After  Kerner 


A\\  B  C  D 

FIG.  87.  Development  of  an  oxalis  leaf 

-4,  full-grown  leaf  ;  B,  rudimentary  leaf,  the  leaflets  not  yet  evident ;  C,  more 
advanced  stage,  the  leaflets  appearing ;  D,  a  still  more  advanced  stage.  B,  C, 
and  Z),  considerably  magnified.  —  After  Frank 


ADVENTITIOUS  BUDS  87 

in  thin  pouches,  which  serve  as  bud  scales,  and  are  thus  entirely 
shielded  from  direct  contact  with  the  outside  air. 

101.  Dormant  buds.    Generally  some  of  the  buds  on  a  branch 
remain  undeveloped  in  the  spring,  when  the  other  buds  are 
beginning  to  grow,  and  this  inactive  condition  may  last  for 
many  seasons.    Finally  the  bud  may  die,  or  some  injury  to  the 
tree  may  destroy  so  many  other  buds  as  to  leave  the  dormant 
ones  an  extra  supply  of  food,  and  this,  with  other  causes,  may 
force  them  to  develop  and  to  grow  into  branches. 

Sometimes  the  tree  altogether  fails  to  produce  buds  at  places 
where  they  would  regularly  occur.  In  the  lilac  the  terminal 
bud  usually  fails  to  appear,  and  the  result  is  constant  forking 
of  the  branches. 

102.  Adventitious  buds.  Buds  which  occur  in  irregular  places, 
that  is,  not  terminal  nor  in  or  near  the  axils  of  leaves,  are  called 
adventitious   buds; 

they  may  spring  from 
the  roots,  as  in  the 
silver-leafed  poplar,  or 
from  the  sides  of  the 

trunk,  as  in  our  Amer- 

-,          T  FIG.  88.  Budding  leaf  of  Bryophyllum 

ican   elm.     In  many 

trees,  for  instance  willows  and  maples,  they  are  sure  to  appear 
after  the  trees  have  been  cut  back.  Willows  are  thus  cut  back, 
or  pollarded,  in  order  to  cause  them  to  produce  a  large  crop  of 
slender  twigs  suitable  for  basket  making. 

Leaves  rarely  produce  buds,  but  a  few  kinds  do  so  when  they 
are  injured.  Those  of  the  Bryopliyllum  (Fig.  88),  a  plant  allied 
to  the  garden  live-forever,  when  they  are  removed  from  the  plant 
while  they  are  still  green  and  fresh,  almost  always  send  out 
buds  from  the  margin.  These  do  not  appear  at  random,  but 
are  borne  at  the  notches  in  the  leaf  margin  and  are  accompa- 
nied almost  from  the  first  by  minute  roots.  This  plant  seems  to 
rely  largely  upon  leaf  budding  to  reproduce  itself,  for  in  a  cool 
climate  it  rarely  flowers  or  seeds. 


CHAPTEE  X 


LEAVES 

103.  The  leaf  as  a  member  of  the  plant  body.  Among  seed 
plants  the  plant  body  consists  of  root  and  shoot.  The  latter 
is  made  up  of  stem  and  leaves.  It  is  diffi- 
cult to  frame  a  simple  and  exact  definition 
for  the  leaf,  but  every  one  is  sufficiently 
familiar  with  the  appearance  of  the  ordi- 
nary foliage  leaves  of  plants,  and  there  is 
no  difficulty  in  identifying  these.  The  un- 
usual scale-like,  bristle-shaped,  tendril- 
shaped,  or  pitcher-form  leaves  are  often 
hard  to  recognize  as  such. 

104.  Parts  of  the  leaf.  In  the  typical 
foliage  leaf  there  are  three  parts,  —  the 
expanded  portion,  or  blade 
(lamina),  the  leafstalk 
(petiole),  and  a  pair  of  ap- 
pendages at  the  base  of  the 

FIG.  89.  Leaf  of  apple,    petiole  known  as  stipules. 
with  stipules  Many   leaves   have  no 

After  Thome  petiole  and  are  said  to  be 

sessile  (meaning  sitting).    Others  have  no  blade 
and  perform  their  functions  as  foliage  by  means 
of  a  flattened  petiole  or  large  stipules.    Most  pIG.  90. 
leaves  are  bilaterally  symmetrical  ;  that  is,  they 
have  a  right  and  a  left  half,  which,  if  folded 
together  along  the  middle  line  of  the  leaf,  would 
nearly  coincide.    Usually  the  upper  and  the  under  surface  differ 
from  each  other  in  color,  smoothness,  and  other  respects. 

88 


Leaf  of 
pansy,  with  leaf- 
like  stipules 

After  Decaisne 


MODES  OF   VEINING 


89 


105.  Veining.    The  blade  of  the  leaf  is  traversed  by  a  frame- 
work   of    tibro-vascular   bundles   known  as   veins.    These    are 
arranged  in  many  ways,  but  the  two  prin- 
cipal types  are  closed,  or  parallel-veined, 

and  open,  or  netted-veined,  leaves.    In  the 
former  the  veins  run  more  or  less  nearly 
parallel,  either  from  base  to  tip  of  the 
leaf,  or  from  a  mid- 
rib outward.    In  the 
latter  the  veins  are 
branched  so  as  to 
form  a  network. 

106.  Palmate  and 
pinnate  veining.    In 
netted-veined  leaves 
several  ribs  may 
radiate  from  the  end 
of  the   petiole,  like 
the  sticks  of  a  fan. 
Such  veining  is  said 
to    be    palmate.     If 

there  is  only  one  midrib,  from  which  smaller  ribs  extend  both 
ways,  the  veining  is  said  to  be  pinnate  (meaning  feather-like). 

Often  the  veining  is  intermediate  be- 
tween these  two  types. 

107.  Relation  of  shape  to  mode  of 
veining.  Since  the  water  supply  of 
the  leaf  is  carried  through  the  veins, 
and  since  they  support  the  softer 
parts  between  them,  one  would  ex- 
pect to  find  that  the  form  of  the  leaf 
would  bear  a  close  relation  to  its 

,-,     no    AT   4  mode  of  veining.    This  is  the  case, 

FIG.  93.    Netted  veining  (pal- 
mate) in  leaf  of  melon         and  m  general  palmately  veined 
After  Decaisne  leaves  are  roundish,  while  pinnately 


FIG.  91.    Parallel-veined 
leaf  of  Solomon's  seal 

After  Strasburger 


FIG.  92.  Parallel  vein- 
ing in  canna.  Veins 
running  from  midrib 
to  margin 


90 


LEAVES 


veined  ones  are  longer  than  they  are  wide.  These  differences 
are  particularly  noticeable  in  leaves  in  which  the  leaf  blade  is 
not  all  of  one  piece,  —  divided  leaves  (Figs.  95,  96). 

Usually  veins,  near  then-  origin,  follow  a  pretty  straight 
course.  This  is  desirable,  in  order  to  carry  water  as  speedily 
as  possible  from  the  base  of  the  leaf  to  its  tip.  The  arrange- 
ment of  the  veins  in  the  leaves  of  most  land 
plants  is  admirably  adapted  to  strengthen 
the  leaf  and  protect  it  from  being  torn. 
In  many  cases  the  last-named  result  is 
secured  by  a  sort  of  "  binding "  of  looped 
veins  running  around  the  margin,  as  is 
fairly  well  shown  in  Fig.  94. 

108.  Description   of   leaf   forms.    The 
various    forms   of   leaves  are   classed   and 
described  by  botanists  with  great  minute- 
ness,1 not  simply  for  the  study  of  leaves 
themselves,  but  also  because  in   classify- 
ing and  describing  plants  the  characteristic 
shapes  of  the  leaves  of  many  kinds  of  plants 
form  a  simple  and  ready  means  of  distin- 

FIG.  94.  Netted  vein-    guishing  them  from  each  other  and  identi- 
ing  (pinnate)  in  leaf    fying  them. 

109.  Occurrence   of   netted  or  parallel 
veining.    With  few  exceptions,  the  leaves 

of  monocotyledonous  plants  are  parallel-veined  and  those  of 
dicotyledonous  plants  netted-veined. 

The  needle-like  leaves  of  the  pines,  spruces,  firs,  larches,  and 
other  coniferous  trees  have  but  a  single  vein,  or  two  or  three 
parallel  ones  ;  but  in  their  case  the  veining  could  hardly  be  other 
than  parallel,  since  the  leaves  are  so  narrow  that  no  veins  of 
any  considerable  length  could  exist  except  in  a  position  length- 
wise of  the  leaf. 


of  foxglove 
After  Planchon 


1  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  I,  pp.  623-637. 
See  also  Appendix  to  this  book. 


SIMPLE  AND  COMPOUND  LEAVES 


91 


Monocotyledonous  plants  seldom  have  leaves  with  notched  or 

cut  margins,  while  dicotyledonous  plants  frequently  have  them. 
A  certain  plan  of  venation  is  found 

mainly  in  plants  with  a  particular  mode 

of  germination,  of  stem  structure,  and  of 

arrangement  of  floral  parts,  and  this  is 

but  one  of  the  frequent  cases  in  botany 

in  which  the  structures  of  plants  are 

correlated  in  a  way  which  is  not  easy 

to  explain. 

No  one  knows  why  plants  with  two 

cotyledons  should  have  netted-veined 

leaves,  but  many  such  facts  as  this  are 

familiar  to  every  botanist. 

110.  Simple  and  compound  leaves. 

The  leaves  so  far  studied  are  simple 

leaves,   that   is,  leaves  of   which   the 

blades  are  more  or  less  entirely  united 
into  one  piece.  But 
while  in  the  elm 
the  margin  is  cut  in 
only  a  little  way,  in 
some  maples  it  is 

deeply  cut  in  toward  the  bases  of  the  veins. 
In  some  leaves  the  gaps  between  the  adja- 
cent portions  extend  all  the  way  down  to 
the  petiole  (in  palmately  veined  leaves)  or 
to  the  midrib  (in  pinnately  veined  ones). 

FIG.  96.  Pinnately  gucn  divided  leaves  are  shown  in  Figs.  95 
divided   leaf   of         -,  n~ 
celandine  and  96' 

Iii  still  other  leaves,  known  as  compound 

After  Decaisne  .   , 

leaves,  or  branched  leaves,  the  petiole,  as 
shown  in  Fig.  99  (palmately  compound),  or  the  midrib,  as  shown 
in  Fig.  97  (pinnately  compound),  bears -what  look  to  be  separate 
leaves.  These  differ  in  their  nature  and  mode  of  origin  from  the 


FIG.  95.    Palmately  divided 
leaf  of  buttercup 

The  blade  of  the  leaf  is  dis- 
continuous, consisting  of 
several  portions.  —  After 
Decaisne 


92 


LEAVES 


portions  of  the  blade  of  a  divided  leaf.  One  result  of  this  differ- 
ence appears  in  the  fact  that  some  time  before  the  whole  leaf  is 
ready  to  fall  in  autumn,  the  leaflets  of  a  compound  leaf  are  seen 
to  be  jointed  at  their  attachments.  In  Fig.  99  the  horse-chestnut 


FIG.  97.  Pinnately  com- 
pound leaf  of  locust, 
with  spines  for  stipules 


FIG.  98.  Pinnately  com- 
pound leaf  of  pea 

A  tendril  takes  the  place 
of  a  terminal  leaflet 


leaf  is  shown  at  the  time  of  falling,  with  some  of  the  leaflets 
already  disjointed.  . 

That  a  compound  leaf,  in  spite  of  the  joints  of  the  separate 
leaflets,  is  really  only  one  leaf  is  shown:  (1)  by  the  Absence  of 
buds  in  the  axils  of  leaflets  (see  Fig.  97) ;  (2)  by  the  horizon- 
tal arrangement  of  the  blades  of  the  leaflets,  without  any  twist 
in  their  individual  leafstalks  ;  (3)  by  the  fact  that  their  arrange- 
ment on  the  midrib  does  not  follow  any  of  the  systems  of  leaf 


COMPOUND  LEAVES 


93 


arrangement  on  the  stem  (Sec.  111).  If  each  leaflet  of  a  com- 
pound leaf  should  itself  become  compound,  the  result  would  be 
to  produce  a  twice  compound  leaf  (Fig.  108). 


FIG.  99.    The  fall  of  the  horse-chestnut  leaf 


CHAPTER  XI 


LEAF  ARRANGEMENT  FOR  EXPOSURE   TO   SUN  AND   AIR; 
HELIOTROPIC  MOVEMENTS  OF  LEAVES  AND  SHOOTS 

111.  Leaf  arrangement.1  Leaves  are  quite  generally  arranged 
so  as  to  secure  the  best  possible  exposure  to  the  sun  and  air. 
This,  in  the  vertical  shoots  of  the  elm,  the  oak  (Fig.  100),  the 
apple,  beech,  and  other  alternate-leaved  trees,  is  quite  consistent 
with  their  spiral  arrangement.  In  horizontal  twigs  and  branches 

MA 


FIG.  100.   Leaf  arrangement 
of  the  oak 


FIG.  101.    Leaf  arrangement 
of  European  beech 


of  the  elm,  the  beech  (Fig.  101),  the  chestnut,  the  linden,  and 
many  other  trees  and  shrubs,  the  desired  effect  is  secured  by  the 
arrangement  of  all  the  leaves  in  two  flat  rows,  one  on  each  side 
of  the  twig.  The  rows  are  produced,  as  is  easily  seen  on  exam- 
ining such  a  leafy  twig,  by  a  twisting  about  of  the  leafstalks. 
The  adjustment  in  many  opposite-leaved  trees  and  shrubs  con- 
sists in  having  each  pair  of  leaves  cover  the  spaces  between 
the  pair  below  it,  and  sometimes  in  the  lengthening  of  the  lower 

i  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  I,  pp.  390-424. 

04 


PLATE  II.   Leaves  arranged  for  maximum  illumination 
After  F.  E.  Clements 


LEAF  MOSAICS 


95 


fe/ , 


leafstalks  so  as  to  bring  the  blades  of  the  lower  leaves  out- 
side those  of  the  upper  leaves.  Examination  of  Figs.  102  and 
103  will  make  the 
matter  clear. 

The  student  who 
observes  the  leafage 
of  trees  of  different 
kinds  on  the  growing 
tree  itself  may  notice 
how  circumstances 
modify  the  position 
of  the  leaves.  Maple 
leaves,  for  example, 
on  the  ends  of  the 
branches  are  ar- 
ranged much  like 
those  of  the  horse- 
chestnut,  but  they 

are  found  to  be  arranged  more  nearly  flatwise  along  the  inner 
portions  of  the  branches,  that  is,  the  portions  nearer  the  tree. 

Figs.  104  and   105 
^  show  the  remarkable 

difference  in  arrange- 
ment  in  different 
^^B^te^v       branches    of    the 
Dcul/iu,  and  equally 
i?     -^ali       interesting     modifica- 
tions may  be  found  in 
alternate-leaved  trees, 
such  as  the  elm  and 
FIG.  103.   Leaf  arrangement  of  horse-chestnut    the  cherry. 


FIG.  102.   Leaf  arrangement  of  horse-chestnut  on 
vertical  shoots  (top  view) 

After  Kerner 


on  vertical  shoots  (side  view) 
After  Kerner 


112.   Leaf  mosaics. 
Jn    very    many    cases 


the  leaves  at  the  end  of  a  shoot  are  so  arranged  as  to  form  a 
rather  symmetrical  pattern,  as  in  the  horse-chestnut  (Fig.  102). 


96 


LEAF  ARRANGEMENT  AND  MOVEMENTS 


When  this  is  sufficiently  regular,  usually  with  the  spaces  between 
the  leaves  a  good  deal  smaller  than  the  areas  of  the  leaves  them- 
selves, it  is  called  a  leaf  mosaic  (Fig.  106).  Many  of  the  most 
interesting  leaf  groups  of  this  sort,  as  in  the  figure  above  men- 
tioned, are  found  in  the  rosettes  of  the  so-called  root  leaves  of 
plants.  Good  examples  of  these  are  the  dandelion,  chicory,  fall 
dandelion,  thistle,  hawkweed,  Pyrola,  and  plantain.  The  leaves 
of  these  plants  are  kept  from  shading  each  other,  sometimes  by 


FIG.  104.   Opposite  leaves  of  Deutzia 1  (from  the  same  shrub  as  Fig.  105) 
as  arranged  on  a  horizontal  branch 

the  narrowness  of  the  leaves  and  sometimes  by  the  lengthening 
of  the  leafstalks  of  the  lower  ones. 

113.  Much-divided  leaves.  Not  infrequently  leaves  are  cut 
into  slender,  fringe-like  divisions,  as  in  the  carrot,  tansy,  south- 
ernwood, wormwood,  yarrow,  dog  fennel,  cypress  vine,  and  many 
other  common  plants.  This  kind  of  leaf  seems  to  be  adapted  to 
offer  considerable  surface  to  the  sun  without  cutting  off  too 
much  light  from  other  leaves  underneath.  Such  a  leaf  is  in 
much  less  danger  of  being  torn  by  severe  winds  than  are  broader 
ones  with  undivided  margins.  The  same  purposes  are  served  by 

1  Deutzia  crenata. 


DAILY  MOVEMENTS  OF  LEAVES 


97 


compound  leaves  with  very 
many  small  leaflets,  such  as 
those  of  the  honey  locust,  the 
mimosa,  acacia  (Fig.  108),  and 
other  trees  and  shrubs  of  the 
pea  family. 

114.  Daily   movements    of 
leaves.    Many   compound 
leaves  have  the  power  of 
changing  the  position  of  their 
leaflets  to  accommodate  them- 
selves to  varying  conditions  of 
light  and  temperature.    Some    FIG.  105.  Opposite  leaves  of  Deutzia, 
plants  have  the  power  of  direct-      as  arra^ed  on  a  vertical  branch  l 
ing  the  leaves  or  leaflets  edgewise  towards  the  sun  during  the 
hottest  parts  of  the  day,  allowing  them  to  extend  their  sur- 
faces more  nearly  in  a  horizontal 
direction  during  the  cooler  hours. 

The  so-called  "sleep"  of  plants 
has  long  been  known,  but  this  sub- 
ject has  been  most  carefully  studied 
rather  recently.  The  wood  sorrel, 
or  oxalis,  the  common  bean,  clovers, 
and  the  locust  tree  are  some  of  the 
most  familiar  of  the  plants  whose 
leaves  assume  decidedly  different 
positions  at  night  from  those  which 
they  occupy  during  the  day.  Some- 
times the  leaflets  rise  at  night,  and 
in  many  instances  they  drop,  as  in  the  red  clover  (Fig.  107) 
and  the  acacia  (Fig.  108).  One  useful  purpose,  at  any  rate,  that 
is  served  by  the  nocturnal  position  of  the  leaf  is  protection 
from  frost.  It  has  been  proved  experimentally  that  when 


FIG.  106.  Leaf  mosaic  of  a 
Campanula 

After  Kerner 


1  It  will  be  noticed  that  the  exposure  to  sunlight  is  here  not  nearly  as 
favorable  as  in  Fig.  104. 


98 


LEAF  ARRANGEMENT  AND  MOVEMENTS 


part  of  the  leaves  on  a  plant  are  prevented  from  assuming  the 
folded  position,  while  others  are  allowed  to  do  so,  and  the  plant  is 

then  exposed  during  a  frosty 
night,  the  folded  ones  may 
escape,  while  the  others  are 
killed.  Since  many  plants  in 
tropical  climates  fold  their 
leaves  at  night,  it  is  certain 
that  this  movement  has  other 

FIG.  107.  A  leaf  of  red  clover  purposes  than  protection 

,  leaf  by  day ;  B,  the  same  leaf  at  night      frorn 


probably 

there  is  much  yet  to  be  learned  about  the  meaning  and  impor- 
tance of  leaf  movements. 

115.  Self-induced  movements ;  sensitive  plants.  Some  leaves, 
notably  those  of  the  so-called  telegraph  plant,1  have  the  power 
of  maintaining  pretty  rapid  movements  without  external  stimuli. 
The  small  lateral 
leaflets  of  this 
plant,  through  a 
cons  iderable 
range  of  temper- 
atures  above 
72°  F.  (22°C.),in 
light  or  darkness 
alike,  continue  to 
move  first  up, 
then  down,  so 
that  their  tips 
make  a  complete  FIG.  108.  A  leaf  of  acacia 

circle  ill  from  one  A,  as  seen  by  day;  B,  the  same  leaf  at  night.  — After 
to  three  or  more  Darwin 

minutes.  The  motion  is  jerky,  like  that  of  the  second  hand  of 
a  watch,  and  gives  one  a  vivid  impression  of  the  plant  as  a 
living  thing. 

1  Desmodium  gyrans. 


MECHANISM  FOR  LEAF  MOTIONS 


99 


A  good  many  plants  of  the  pea  family  have  leaflets  which  are 
sensitive  to  the  touch.  The  best-known  species  is  the  common 
sensitive  plant  of  the  florists,1  the  leaflets  of  which  close  and 
drop,  like  those  of  Fig.  108,  and  the  leafstalks  droop  when  the 
plant  is  touched  or  jarred.  Some  of  our  common  wild  plants  of 
the  same  family  2  have  leaves  which  promptly  show  irritability 

when  touched,  and  one  species  is 
locally  known  as  "shame  vine," 
from  this  peculiarity. 

116.  Structure  of  the  parts 
which  cause  leaf  motions.  In  a 
great  number  of  cases  the  daily 
movements  of  leaves  are  produced 
by  special  organs  at  the  bases  of 
the  leafstalks.  These 
cushion-like  organs, 
called  pulvini  (Fig. 
JjL  109),  are  composed 
!j  mainly  of  paren- 
|  chymatous  tissue, 
\\  which  contains  much 
water.  It  is  impossi- 
ble fully  to  explain  in 
simple  language  the  way  in  which  the  cells  of  the  pulvini  act,  but 
in  a  general  way  it  may  be  said  that  changes  in  the  light  to  which 
the  plant  is  exposed  cause  rather  prompt  changes  in  the  amount 
of  water  in  the  cells  in  one  portion  or  other  of  the  pulvinus.  If 
the  cells  on  one  side  are  filled  fuller  of  water  than  usual,  that 
side  of  the  pulvinus  will  be  expanded  and  make  the  leafstalk 
bend  toward  the  opposite  side.  The  promptness  of  these  move- 
ments is  no  doubt  in  considerable  measure  due  to  the  fact  that 
in  the  pulvini,  as  in  many  other  parts  of  plants,  the  protoplasm 
of  adjacent  cells  is  connected.  Delicate  threads  of  protoplasm 
extend  through  the  cell  walls,  making  the  whole  tissue  a  living 
1  Mimosa  pudica.  '2  Species  of  Cassia  and  Desmanthus. 


FIG.  109.    Compound  leaf  of  bean  with 
pulvinus 

The  pulvinus  shows  as  an  enlargement  in 
the  figure  about  three-eighths  inch  long, 
at  the  base  of  the  petiole.  —After  Sachs 


100 


LEAF  ARRANGEMENT  AND  MOVEMENTS 


web,  so  that  any  suitable  stimulus  or  excitant  which  acts  on 
one  part  of  the  organ  will  soon  affect  the  whole  organ. 

117.  Vertically  placed  leaves.    Many  leaves,  like  those  of 
the  olive  (Fig.  Ill),  always  keep  their  principal  surfaces  nearly 


FIG.  110.   Leaves  standing  nearly  vertical  in  compass  plant  (Silphium 
laciniatum) 

A,  view  from  east  or  west ;  B,  from  north  or  south.  —  After  Kerner 

vertical.  Thus  they  receive  the  morning  and  evening  sun  upon 
their  faces,  and  the  noonday  sun  (which  is  so  intense  as  to 
injure  them  when  received  full  on  the  surface)  upon  their  edges. 


HELIOTROPIC  MOVEMENTS:"     ./>,  \     \ 

This  adjustment  is  most  perfect  in  the  compass  plant  of  the 
prairies  of  the  Mississippi  basin.  Its  leaves  stand  nearly  upright, 
many  with  their  edges  just  about  north  and  south  (Fig.  110),  so 
that  the  rays  of  the  midsummer  sun  will,  during  every  bright 
day,  strike  the  leaf  surfaces  nearly  at  right  angles  during  a 
considerable  portion  of  the  forenoon  and  afternoon,  while  at 
midday  only  the  edge  of  each  leaf  is  exposed  to  the  sun. 


FIG.  111.  Nearly  vertical  leaves  of  the  olive 


118.  Helio tropic  movements.    The  whole  plant  above  ground 
usually  bends  toward  the  quarter  from  which  most  light  comes. 

Any  set  of  flowering  plants  growing  close  to  a  wall,  or  of 
house  plants  in  a  window,  generally  offers  many  illustrations  of 
this  principle.  Movements  caused  by  light  are  called  heliotropic 
movements  (from  two  words  meaning  turning  toward  light). 

119.  Positive   and  negative   heliotropic   movements ;    how 
produced.    Plants  may  bend  either  toward  or  away  from  the 
strongest  light.    In  the  former  case  they  are  said  to  show  posi- 
tive Jieliotropism,  in  the  latter  negative  heliotropism.    In  both 
cases  the  movement  is  produced  by  unequal  growth,  brought 
about  by  the  stimulus  of  unequal  lighting  of  different  sides  of 
the  stem.    A  plant  if  placed  on  a  revolving  table  before  a  window 
and  slowly  turned  during  the  hours  of  daylight  grows  upright, 
like  a  plant  out  of  doors.    This  is  because  it  is  not  left  with  a 
one-sided  illumination  long  enough  to  produce  any  bending. 


• 


CHAPTER   XII 
MINUTE  STRUCTURE  OF  LEAVES;  FUNCTIONS  OF  LEAVES* 

120.  Outline  of  leaf  structure.    Most  foliage  leaves  of  seed 
plants  contain   a  rather  complicated  system  of  nbro-vascular 
bundles  forming  the  veins  (Sec.  105),  which,  taken  together,  con- 
stitute a  framework  by  which  the  leaf  is  supported  and  strength- 
ened.   Over  and  around  these  veins  lies  a  mass  of  green  pulpy 
material,  the  spongy  parenchyma. .  The  whole  leaf  is  covered  by 
an  epidermis.    Frequently,  especially  in  soft  and  rather  thick 
leaves,  such  as  those  of  the  garden  live-forever,  the  epidermis 
can  be  readily  peeled  off  as  a  thin,  transparent  skin. 

The  epidermis  and  the  spongy  parenchyma  decay  far  more 
readily  than  the  woody  framework,  and  so  skeleton  leaves  may 
often  be  found  on  the  ground  in  the  spring,  showing  plainly  the 
arrangement  of  the  veins  of  the  leaf. 

121.  Details  of  a  leaf  section.  .The  relative  positions  and 
the  detailed  structure  of  the  parts  mentioned  in  Sec.  120  are 
best  understood  by  reference  to  the  magnified  cross  section  of  a 
typical  foliage  leaf. 

In  the  ordinary  leaf  (Fig.  112)  a  section  shows  at  the  upper 
surface  a  layer  of  transparent  cells  of  the  epidermis  e,  '  Beneath 
this  lies  a  layer  of  elongated  cells  p,  of  a  green  color,  standing 
at  right  angles  to  the  epidermis.  These  are  called  palisade  cells, 
from  a  fancied  resemblance  of  their  shape  and  relative  position 
to  palisades.  Under  this  layer  the  leaf  interior  is  filled  with  an 
irregularly  grouped  mass  of  green  cells  known  as  the  spongy 
parenchyma  sp,  throughout  which  occur  numerous  air  spaces  a, 

*  To  THE  INSTRUCTOR  :  As  the  present  chapter  takes  up  its  topics  in  con- 
siderable detail,  it  is  suggested  that  it  may  be  found  expedient,  if  time  is 
limited,  to  omit  Sees.  129,  130,  132,  134,  139  (table),  145-147. 

102 


DETAILS  OF  LEAF  STRUCTURE 


103 


and  in  which  is  an  occasional  nbro-vascular  bundle  I.  The 
palisade  layer  or  layers  and  the  spongy  parenchyma  are  together 
known  as  mesophyll  (meaning  middle  of  leaf). 

The  lower  surface  of  the  leaf  is  covered  by  a  layer  of  color- 
less epidermal  cells  e',  differing  somewhat  in  size  and  shape  from 
those  of  the  upper  epidermis. 

The  lower  epidermis  is  pierced  by  many  openings  or  stomata  s. 
Each  stoma  opens  into  an  air  chamber.  The  upper  epidermis 


FIG.  112.   Cross  section  of  privet  leaf 

e,  upper  epidermis;  p,  palisade  cells;  sp,  spongy  parenchyma;  a,  air  spaces; 
6,  fibre-vascular  bundle;  ef,  lower  epidermis;  5,  stoma.  Much  magnified. — 
Modified  after  Giesenhagen 

of  this  leaf  contains  far  less  stomata  than  the  lower  one,  and 
this  is  true  of  most  leaves,  —  often  the  upper  surface  contains 
none. 

122.  Uses  of  the  parts  above  mentioned.  It  will  be  most 
convenient  to  discuss  the  uses  of  the  parts  of  the  leaf  in  detail 
a  little  later,  but  it  will  make  matters  simpler  to  state  at  once 
that  the  epidermis  serves,  as  a  mechanical  protection  to  the  parts 
beneath  and  prevents  excessive  evaporation ;  that  the  palisade 
cells  hold  large  quantities  of  the  green  coloring  matter  of  the 
leaf  in  a  position  where  it  can  receive  enough  but  not  too  much 
sunlight ;  and  that  the  cells  of  the  spongy  parenchyma  share  the 


104 


STRUCTURE  AND  FUNCTIONS  OF  LEAVES 


-work  of  the  palisade  cells,  besides  evaporating  much  water.  The 
stomata  admit  air  to  the  interior  of  the  leaf,  where  the  air  spaces 
serve  to  store  and  to  distribute  it ;  they  allow  oxygen  and  carbon 
dioxide  gas  to  escape ;  and,  above  all,  they  regulate  the  evapora- 
tion of  water  from  the  plant. 

123.  The  epidermis.  The  cells  of  the  epidermis  are  very  gen- 
erally rilled  with  water.  Their  form  and  the  thickness  and 
material  of  their  walls  depend  largely  on  the  kind  of  soil  and 


FIG.  113.   Surface  view  of  the  epidermis 
of  a  buttercup  leaf l 

e,  cells  of  epidermis ;  n,  nuclei  of  epidermal 
cells ;  #,  guard  cell  of  stoma ;  s,  stoma. 
Much  magnified.  —  After  Giesenhagen 


FIG.  114.  Section  through 
stoma  of  a  buttercup 
leaf,  at  right  angles  to 
epidermis 

e,  epidermal  cells;  g,  guard 
cell  of  stoma  ;  s,  stoma  ; 
ch,  air  chamber.  Much 
magnified. — After  Bonnier 
and  Sablon 


climate  to  which  the  plant  ^is  adapted.  In  most  herbs  the  epi- 
dermal cells  form  only  a  single  layer  and  are  not  greatly 
thickened. 

The  stomata  are  not  mere  holes  in  the  epidermis,  but  have  a 
somewhat  complicated  structure.  Each  stoma  consists  of  two 
kidney-shaped  guard  cells  inclosing  a  slit-like  opening  into  the 
leaf  (Fig.  113). 

When  the  stoma  is  viewed  in  a  section  at  right  angles  to  the 
surface  of  the  leaf  (Fig.  114)  it  appears  as  a  narrow  passage 
communicating  with  an  air  chamber  inside  the  epidermis. 

The  number  of  stomata  in  a  square  inch  of  leaf  surface  is 
very  great.  An  apple  leaf  contains  about  24,000  and  a  black 


1  Fig.  113  is  from  Ranunculus  Ficaria ;  Figs.  114-118  from  E.  acris. 


CHLOROPLASTS   AND  CHLOROPHYLL 


105 


FIG.  115.   Upper  epidermis  and  palisade  cells  of 
a  buttercup  leaf 

A,  section  perpendicular  to  upper  surface;  B,  exte- 
rior view  of  upper  surface  with  palisade  cells  seen 
through  epidermis  ;  e,  epidermis  ;  p,  palisade  cells. 
Much  magnified.  —  After  Bonnier  and  Sablon 


walnut  leaf  about 
300,000  per  square 
inch  of  the  lower 
epidermis. 

124.  The  meso- 
phyll ;  chloroplasts ; 
chlorophyll.  The 
mesophyll  appears  to 
the  naked  eye  of  a 
uniform  green,  but 
under  the  microscope 
its  cells  are  seen  to 
contain  many  green 
structures  called  chlorophyll  bodies  or  chloroplasts  (" chlorophyll" 
meaning  leaf  green  and  "  chloroplast "  meaning  molded  out  of 
green  material).  The  color  of  the  leaf,  as  well  as  that  of  green 
stems  and  other  parts  of  the  plant  body,  is  due  to  these.  A 

chloroplast  is  usually,  in  seed  plants  and 

in  the  higher  spore  plants,  of  an  ellipsoidal 

form  or  lens-shaped 

a-nd   somewhat 

translucent.    Its 

color   is  due    to   a 

green  substance, 

FIG    116.   Passage  of  a    soM)le    in    alcohol 
fibro-vascular    bundle 

from  stem  to  leaf  of     b^t    not    in    water, 
a  buttercup  (diagram-    known     as     cllloro- 


matic) 


phyll. 


FIG.  117.  Diagram  of 


125.  Woody  tis-     distribution  of  fibro- 

vascular  bundles  in  the 

sue  in  leaves.    The      leafstalk  of  a  buttercup 
veins  of  leaves  con- 


s,  stem;  10,  woody  part  of 
bundle  ;  b,  sieve  cells  of 
bundle.  —  After  Bonnier 
and  Sablon 

e,  epidermis  ;  iv,  woody 
part  of  bundle;  b,  sieve 
cells  of  bundle  ;  /,  fibrous 
layer  on  outer  part  of 

of  the  stem  of  the  plant.    Indeed,  these     bundle.   Magnified.— 

,,,..,,„  ,.  .,, 

bundles  m  the  lear  are  continuous  with 


sist  of  fibro-vascular  bundles  containing 
wood  fibers  and  vessels  much  like  those 


After   Bonnier  and    Sa- 


106 


STRUCTURE  AND  FUNCTIONS   OF  LEAVES 


those  of  the  stem  and  consist  merely  of  portions  of  the  latter 
which  pass  outward  and  upward  from  the  stem  into  the  leaf 
under  the  name  of  leaf  traces. 

The  manner  in  which  fibre-vascular  bundles  pass  from  the 
stem  through  the  petiole  into  the  leaf  and  are  there  distributed 
can  readily  be  gathered  from  an  examination  of  Figs.  116—118. 
Their  wood  cells  and  vessels  serve  to 
carry  water  into  the  leaf,  while  their 
sieve  cells  carry  plant  food  from  its 
place  of  manufacture  in  the  blade  of 
the  leaf  down  into  the  stem. 


FIG.  118.  Part  of  the  fibro- 
vascular  skeleton  of  a  but- 
tercup leaf 

Much  magnified.  —  After  Bon- 
nier and  Sablon 


FIG.  119.  Termination 
of  a  vein  in  a  leaf 

v,  spirally  thickened 
cells  of  the  vein;  p, 
parenchyma  cells  of 
the  spongy  interior 
of  the  leaf,  with  chlo- 
rophyll bodies ;  n,  nu- 
cleated cells.  x  about  345  diameters 


126.  Nutrition.  The  series  of  processes  by  which  the  plant 
(1)  takes  up  the  raw  materials  to  form  its  food,  (2)  unites  these 
into  foods,  and  finally  (3)  constructs  tissue  from  these  foods,  or 
(4)  stores  them,  constitutes  nutrition. 

A  good  deal  of  that  portion  of  nutrition  included  under 
(1)  is  carried  on  by  the  roots.  But  all  kinds  of  nutritive  work 
are  carried  on  in  green  leaves,  and  the  portion  numbered  (2)  is 
a  specialty  of  green  plant  cells,  particularly  of  those  in  leaves. 


PHOTOSYNTHESIS  107 

127.  The  work  of  leaves.   A  leaf  has  four  principal  functions: 

1.  Photosynthesis.  3.  Assimilation. 

2.  Respiration.  4.  Transpiration. 

128.  Photosynthesis.    All  green  leaves,  when  in  healthy  con- 
dition, at  suitable  temperatures  and  with  sufficient  illumination 
can  produce  carbohydrates  (starch  or  sugars}  from  carbon  dioxide 
and  water. 

This  process  is  of  the  greatest  importance,  since  directly  or 
indirectly  all  plants  and  animals  depend  upon  it  for  their  food 
supply.  The  manufacture  by  the  plant  of  carbohydrates  from 
the  raw  materials  is  known  as  photosynthesis  (from  two  words 
meaning  light  and  putting  together).  It  is  often  called  fixation 
of  carbon  or  assimilation  of  carbon.  Photosynthesis  is  per- 
formed by  the  chloroplasts,  especially  in  the  palisade  cells,  and 
goes  on  imperfectly  or  not  at  all  in  plants  or  parts  of  plants, 
as  in  certain  parasites  and  other  forms,  in  which  no  chlorophyll 
exists  (Chapters  xxn,  xxx). 

129.  Chemical  formula  for  photosynthesis.    The  details  of 
the  photosynthetic  process  are  not  wholly  known,  and  it  is  not 
at  all  likely  that  in  starch-producing  plants  starch  is  the  first 
substance  formed  from  carbon  dioxide  and  water,  but  it  is  one 
of  the  early  products  of  the  action  of  the  chloroplasts  and  is 
the  easiest  to  detect  by  chemical  tests  applied  to  the  leaf.    In 
some  plants,  as  the  onion,  the  products  of  photosynthesis  are 
all  stored  in  the  form  of  sugar. 

If  the  chloroplast  produced  starch  as  the  direct  result  of 
combining  carbon  dioxide  and  water,  the  chemical  equation  for 
the  process  would  in  its  simplest  form  be J : 


I  Six  molecules 


Six  molecules  1        f  Five   mole-  "1         f  One    mole- 

of     carbon  L  +  J       cules     of  L  =  J       cule       of  >  -t-  <         f 
dioxide         J        [^      water        J        [^      starch       J         [ 

6C02  +          5H20          -        C6H1005        +  602 

1  Really  some  multiple  of  C6Hi005  probably  more  nearly  expresses  the 
composition  of  starch  than  the  simple  formula  given.  It  is  certain  that  the 
photosynthetic  process  is  much  more  complicated  than  a  mere  combination 
of  carbon  dioxide  with  water  to  form  either  starch  or  sugar. 


108          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

If  glucose  (grape  sugar)  were  the  first  product,  the  simplest 
equation  would  be : 

6  CO2  +  6  H2O  =  C6H12O6  (one  molecule  of  glucose)  +  6  O,,.1 

It  should  be  noticed  that  each  of  the  processes  above  formu- 
lated results  in  the  disappearance  of  six  molecules  of  carbon 
dioxide  and  the  production  of  six  molecules  of  oxygen  as  a 
waste  product. 

These  facts,  namely,  that  in  the  green  parts  of  plants  exposed 
to  sunshine  carbon  dioxide  is  consumed  and  oxygen  liberated, 
form  the  foundation  of  our  knowledge  of  photosynthesis.  The 
first  step  in  the  study  of  the  subject  was  taken  by  Joseph 
Priestley  in  1771,  by  his  discovering  that  air  in  which  candles 
had  been  burned  until  they  went  out  could  be  restored  to 
something  like  its  original  condition  by  leaving  in  it  for  some 
time  vigorous  leafy  sprigs  of  mint. 

130.  External  conditions  for  photosynthesis.  Photosynthesis 
can  only  occur : 

1.  When  the  plant  is  supplied  with  air  containing  carbon  dioxide. 

2.  When  the  temperature  is  neither  too  high  nor  too  low. 

3.  When  the  illumination  is  sufficient. 

Ordinary  air  contains  about  one  twenty-fifth  of  one  per  cent 
of  its  bulk  of  carbon  dioxide.  An  increase  of  this  amount  up 
to  four  per  cent,  or  one  hundred  times  the  normal  quantity,  in- 
creases photosynthesis,  but  a  larger  proportion  usually  at  length 
proves  injurious  to  the  health  of  the  plant. 

Some  arctic  and  alpine  plants  can  perform  the  work  of  mak- 
ing carbohydrates  at  temperatures  as  low  as  the  freezing  point 
of  water,  but  plants  of  warmer  climates  require  a  higher  tem- 
perature. The  rate  of  photosynthesis  usually  increases  with 
rise  of  temperature  up  to  about  77°  F.  (25°  C.),  after  which 
it  decreases. 

Photosynthesis  may  go  on  very  feebly,  even  in  compara- 
tive darkness,  but  the  light  of  the  interior  of  ordinary  rooms  is 

1  See  Peirce,  Plant  Physiology,  pp.  58-66. 


FORMATION  AND  ACTION  OF  CHLOROPHYLL      109 

insufficient  for  the  vigorous  growth  of  most  seed  plants  except- 
ing those  which,  in  a  wild  condition,  flourish  in  the  shade.  The 
rate  of  photosynthesis  for  most  of  the  higher  plants  increases 
with  the  illumination  up  to  a  light  intensity  equal  to  that  of 
full  sunlight. 

131.  Conditions  for  formation  of  chlorophyll;  its  mode  of 
action.    Chlorophyll  is  usually  produced  only  in  plants  grown 
in  the  light.    Seedlings  which  have  been  sprouted  in  total  dark- 
ness almost  always  have  a  white  or  very  pale  yellow  color,  and 
blanched  celery  affords  a  familiar  example  of  the  appearance  of 
leaves  grown  in  comparative  darkness.    Microscopical  examina- 
tion of  thoroughly  blanched  plants  shows  them  to  be  destitute 
of  any  decidedly  green  chloroplasts,  and  alcohol  fails  to  extract 
from  them  the  green  chlorophyll  solution  which  is  readily  ob- 
tained from  ordinary  leaves. 

Iron  must  be  present  in  the  soil  in  order  to  enable  the  plant 
to  form  chlorophyll,  and  plants  developed  in  water  cultures  abso- 
lutely free  from  iron  remain  yellow  and  grow  feebly. 

Chlorophyll  appears  to  act  by  intercepting  a  considerable 
portion  of  the  light  rays  which  strike  the  leaf,  thus  compelling 
them  to  expend  their  energy  on  the  chloroplasts  and  so  to  pro- 
duce photosynthesis.  If  light  traverses  a  substance  with  great 
ease,  as  it  does  pure  dry  air,  for  example,  comparatively  little 
effect  is  produced.  On  the  other  hand,  when  it  strikes  a  sub- 
stance which  readily  absorbs  it,  heating  or  chemical  effects  or 
both  are  produced,  as  is  evident  when  a  rough  sheet  of  iron,  a 
sensitized  photographic  dry  plate,  or  blue-print  paper  is  exposed 
to  sunlight.  Chlorophyll  cannot  itself  do  the  work  of  photo- 
synthesis, but  it  causes  the  light  rays  to  act  on  the  chloro- 
plasts so  that  their  protoplasm  carries  on  the  manufacture  of 
carbohydrates  from  the  raw  materials. 

132.  Rate  of  starch  making.    The  amount  of  starch  manu- 
factured daily  by  a  given  area  of  foliage  must  depend  on  the 
kind  of  leaves,  the  temperature  of  the  air,  the  intensity  of  the 
sunlight,  and  some   other    conditions.     Sunflower   leaves  and 


110          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

pumpkin  or  squash  leaves  produce  starch  at  about  the  same  rate. 
In  a  summer  day  fifteen  hours  long  they  can  make  nearly  three 
quarters  of  an  ounce  for  each  square  yard  of  leaf  surface.  A 
full-grown  squash  leaf  has  an  area  of  about  one  and  one-eighth 
square  feet,  and  a  plant  may  bear  as  many  as  a  hundred  of  them. 
The  entire  plant  would  then  produce  nearly  nine  and  a  half 
ounces  of  starch  per  day. 

Another  way  to  emphasize  the  amount  of  work  done  by  the 
leaves  is  to  consider  how  much  air  would  be  needed  to  supply 
the  carbon  in  a  given  weight  of  wood ;  for  all  this  carbon  has 
probably  been  derived  from  carbohydrates  made  in  the  leaves 
(or  other  green  parts)  by  photosynthesis.  If  the  wood  of  a  tree 
after  drying  weighs  11,000  pounds  and  is  half  carbon,  the  latter 
would  weigh  5500  pounds.  Taking  the  carbon  dioxide  contents 
of  the  air  at  ^m,  there  would  be  more  than  20,000,000  cubic 
yards  of  air  needed  to  furnish  the  carbon  of  such  a  tree.1 

The  enormous  amounts  of  carbon  dioxide  annually  removed 
from  the  air  by  the  growth  of  plants  are  continually  being 
replaced  by  the  respiration  of  animals,  the  decay  of  animal  and 
vegetable  material,  and  by  the  burning  of  fuel.  From  the  burn- 
ing of  coal  alone  it  is  estimated  that  nearly  3,000,000  million 
pounds  of  carbon  dioxide  are  every  year  returned  to  the 
atmosphere. 

133.  Respiration.  Plants  cannot  carry  on  their  life  processes 
without  consuming  oxygen  and  giving  off  carbon  dioxide  and 
water.  This  oxygen  consumption  is  the  respiration  of  plants. 
Like  animals,  plants  are  dependent  on  the  union  of  oxygen  with 
oxidizable  substances  in  their  tissues  for  the  energy  with  which 
they  do  the  work  of  assimilation,  growth,  and  reproduction,— 
in  other  words  perform  their  life  processes. 

How  oxygen  can  be  made  to  combine  with  the  carbon-  and 
hydrogen-containing  compounds  in  the  plant  at  moderate  tem- 
peratures is  a  problem  which  plant  physiologists  have  not  yet 
fully  solved ;  but  the  union  does  constantly  go  on,  and  as  a 

1  Taken  with  slight  alterations  from  Peirce,  Plant  Physiology,  p.  44. 


PLATE  III.   A  cypress  swamp,  the  trees  draped  with  Spanish  moss 

(Tillandsia) 
Modified,  after  H.  J.  Webber 


RESPIRATION 


111 


result  of  the  combination,  water  and  carbon  dioxide  are  con- 
tinually excreted. 

The  amount  of  oxygen  absorbed  and  of  carbon  dioxide  given 
off  is,  however,  so  trifling  compared  with  the  amount  of  each 
gas  passing  in  the  opposite  direction,  while  starch  making  is 
going  on  in  sunlight  at  temperatures  most  favorable  for  photo- 
synthesis, that  under  such  circumstances  it  is  difficult  to  observe 
the  occurrence  of  respiration. 

When  the  illumination  is 
very  feeble,  from  -^  to  ^ 
that  of  bright,  diffuse  day- 
light, the  manufacture  of 
carbon  dioxide  by  respira- 
tion and  its  consumption  by 
photosynthesis  are  equal. 

At  high  temperatures, 
such  as  104°  F.  (40°  C.), 
respiration  may  produce  car- 
bon dioxide  more  rapidly 
than  photosynthesis  can  con- 
sume it,  even  with  brilliant 
illumination. 

In  ordinary  leafy  plants 
the  leaves,  through  their 
stomata,  are  the  principal  organs  for  absorption  of  air,  but  much 
air  passes  into  the  plant  through  the  lenticels  of  the  bark. 

In  partly  submerged  aquatics  especial  provisions  are  found 
for  carrying  the  air  absorbed  by  the  leaves  down  to  the  sub- 
merged parts.  This  is  accomplished  in  pond  lilies  by  ventilating 
tubes  which  traverse  the  leafstalks  lengthwise.  In  many  cases 
such  channels  run  up  and  down  the  stem  (Fig.  120).  In  the 
American  cypress  (Taxodium)  the  "knees,"  which  rise  from  the 
roots,  as  shown  in  Plate  III,  are  thought  to  be  for  use  in  respira- 
tion, obtaining  oxygen  from  the  air  and  carrying  it  into  the 
roots  beneath  the  water. 


FIG.  120.   Cross  section  of  stem  of  mares- 
tail  (Hippuris),  with  air  passages  a 

After  Baillon 


112          STRUCTURE  AND  FUNCTIONS   OF  LEAVES 

134.  Resting   condition   and    diminished   respiration.    The 

whole  plant  body  or  parts  of  it  may  pass  into  a  resting  condi- 
tion, in  which  growth  is  suspended  and  few  manifestations  of  life 
are  discernible.  Familiar  examples  of  this  inactive  condition  are 
leafless  trees  in  winter,  and  rootstocks,  tubers,  and  bulbs  during 
the  winter  of  ordinary  temperate  climates  or  the  rainless  sum- 
mer of  southern  California  and  the  Mediterranean  coast  region. 
Seeds  and  many  kinds  of  resting  spores  afford  extreme  instances 
of  the  possibility  of  a  suspension  of  activity  for  years,  followed 
by  prompt  growth  when  suitable  conditions  are  supplied.  In 
general,  a  moderately  low  temperature  and  dryness  favor  the 
resting  state.  During  the  resting  period  respiration  is  greatly 
diminished,  so  much  so  in  the  case  of  thoroughly  dry  seeds  as 
to  be  almost  or  quite  imperceptible. 

When  resting  protoplasm  is  placed  in  circumstances  which 
enable  it  to  begin  active  respiration,  growth  and  development 
soon  appear.  Thus  twigs  of  lilac  or  other  shrubs  will  flower 
after  a  time,  when  placed  in  water  and  brought  into  a  warm 
room  in  winter. 

In  many  cases,  as  with  most  seeds,  the  period  of  repose  is 
essential-  for  growth.  Potato  tubers  will  not  sprout  as  soon  as 
they  are  mature:  some  varieties  need  only  two  months  and 
others  four  or  five  months  of  rest. 

135.  Assimilation.    By  most  American  plant  physiologists l 
the  word  assimilation  is  used  as  a  name  for  the  series  of  changes 
by  which  the  plant  transforms  absorbed  or  manufactured  food 
into  the  materials  of  its  own  tissues. 

The  transformation  of  starch  or  sugar  into  substances,  like 
cellulose,  which  consist  of  the  same  elements  (carbon,  hydrogen, 
and  oxygen)  differently  combined,  is  a  relatively  simple  matter ; 
but  the  manufacture  from  carbohydrates  of  such  very  complex 
nitrogenous  substances  as  the  proteids  and  living  protoplasm 
is  a  most  complicated  process,  and  imperfectly  understood. 

1  European  botanists  often  include  in  the  term  assimilation  both  photo- 
synthesis and  the  processes  discussed  in  this  section. 


PHOTOSYNTHESIS  AND  RESPIRATION 


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114          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

Probably  diastase  or  some  other  ferment  in  the  green  parts  of 
the  plant  transforms  the  newly  made  starch  into  sugar,  and  some 
of  this  is  apparently  combined  on  the  spot  with  nitrogen,  sul- 
phur, and  phosphorus.  These  elements  are  derived  from  nitrates, 
sulphates,  and  phosphates,  taken  up  in  a  dissolved  condition 
by  the  roots  of  the  plant  and  transported  to  the  leaves.  The 
details  of  the  process  are  not  understood,  but  the  result  of  the 
combination  of  the  sugars  or  similar  substances  with  suitable 
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is  to  form  complex  nitrogen  compounds.  These  are  not  precisely 
of  the  same  composition  as  the  living  protoplasm  of  plant  cells 
or  as  the  reserve  proteids  stored  in  seeds  (Sees.  8,  12),  stems 
(Sec.  66),  and  other  parts  of  plants,  but  are  readily  changed 
into  protoplasm  or  proteid  foods  as  necessity  may  demand. 

Assimilation  is  by  no  means  confined  to  leaves ;  indeed,  most 
of  it,  as  above  suggested,  must  take  place  in  other  parts  of  the 
plant.  For  instance,  the  manufacture  of  the  immense  amounts 
of  cellulose,  of  cork,  and  of  the  compound  (lignin)  characteristic 
of  wood  fiber,  which  go  to  make  up  the  main  bulk  of  a  large  tree, 
must  be  carried  on  in  the  roots,  trunk,  and  branches  of  the  tree. 

137.  Metabolism.  It  is  convenient  to  have  a  single  word  to 
express  all  the  chemical  changes  which  are  controlled  by  the 
living  protoplasts.  Such  a  word  is  metabolism.  It  embraces  all 
the  nutritive  processes  mentioned  in  Sec.  126,  as  well  as  respi- 
ration and  the  chemical  changes  concerned  in  the  excretion  of 
waste  materials. 

There  are  two  principal  types  of  metabolic  processes,  —  con- 
structive metabolism  (such  as  photosynthesis),  which  unites 
simpler  compounds  into  more  complex  ones,  and  destructive 
metabolism  (such  as  respiration),  which  breaks  up  complex 
substances  into  simpler  ones. 

Digestive  metabolism,  performed  by  means  of  various  ferments, 
begins,  as  already  mentioned,  in  the  seed  during  germination  and 
is  carried  on  in  most  parts  of  the  higher  plants  during  all  active 
periods  of  their  lives.  It  is  especially  energetic  in  removing 


SUMMARY  OF  METABOLIC  AND  OTHER  PROCESSES     115 


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116          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

the  newly  formed  starch  from  the  green  cells  of  leaves  for  use 
in  other  parts  of  the  plant  body.  Much  of  this  food  (carried 
about  in  the  form  of  a  solution  of  sugar)  is  used  for  building 
material,  as  suggested  in  Sec.  136 ;  but  a  good  deal  of  it  is  often 
transported  to  parenchyma  cells  of  the  stem  and  the  roots, 
where  it  is  changed  back  into  starch  for  storage.  This  change 
is  accomplished  by  small  structures  known  as  leucoplasts  in  the 
cells.  Each  leucoplast  may  cause  a  deposit,  upon  some  part  of 
its  outer  surface,  of  successive  layers  which  finally  develop  into 
a  complete  starch  grain.  How  the  leucoplast  is  able  to  bring 
about  the  change  from  starch  to  sugar  is  unknown. 

139.  Transpiration.  The  process  of  giving  off  water  in  the 
form  of  vapor  from  the  stomata  of  plants  is  called  transpiration. 
It  is  not  a  mere  drying  up,  such  as  occurs  when  a  pile  of  sea- 
weeds or  a  split  stick  of  cord  wood  is  exposed  to  dry  air,  but  is 
an  important  function  of  the  leaves  of  most  seed  plants  and  of 
the  higher  spore  plants.  In  such  forms  as  the  cactuses  (Fig.  50), 
which  are  practically  leafless,  transpiration  is  performed  by  the 
epidermis  of  the  stem. 

As  already  mentioned  (Sec.  36),  ordinary  terrestrial  seed  plants 
are,  during  the  active  periods  of  their  lives,  continually  absorbing 
water  through  the  roots.  This  water  brings  with  it  dissolved 
salts  from  the  soil,  many  of  which  are  used  in  the  tissue- 
forming  work  of  the  plant  body.  Some  of  the  water,  but  only 
an  insignificant  portion  of  the  wliole  amount,  is  needed  for 
photosynthesis,  and  a  good  deal  of  it  is  useful  in  carrying  the 
soluble  plant  foods,  such  as  sugars,  to  the  growing  parts;  but 
there  remains  a  large  excess  of  water  to  be  excreted,  and  this 
duty  is  mainly  performed  by  the  mesophyll,  and  its  amount  is 
regulated  by  the  epidermis  of  the  leaves.  The  air  within  the 
intercellular  spaces  of  the  mesophyll  is  surrounded  by  thin- 
walled  cells  filled  with  watery  protoplasm,  and  it  must  there- 
fore be  nearly  or  quite  saturated  with  moisture.  When  allowed 
to  escape  from  the  leaf  this  air  rapidly  carries  off  quantities  of 
watery  vapor. 


USES  OF  THE   EPIDERMIS 


117 


140.  Uses  of  the  epidermis.1  The  epidermis,  by  its  tough- 
ness, tends  to  prevent  mechanical  injuries  to  the  leaf.  After 
the  change  of  the  outer  portions  of  its  cell  walls  into  a  corky 
substance  it  greatly  diminishes  evaporation  from  the  general 
surface.  This  process  of  becoming  filled  with  cork  material, 
suberin  (or  a  substance  of  similar  properties  known  as  cutin),  is 
essential  to  the  safety  of  leaves  or  of  young  stems  which  have 
to  withstand  heat  and  dryness.  The  corky  or  cutinized  cell 
wall  is  waterproof,  while  ordinary  cellulose  allows  water  to 
soak  through  it  with  ease. 
Merely  examining  sec- 
tions of  the  various  kinds 
of  epidermis  will  not  give 
nearly  as  good  an  idea  of 
their  properties  as  can  be 
obtained  by  studying 
during  severe  droughts 
the  behavior  of  plants 
which  have  strongly  cuti- 
nized surfaces  and  of 
those  which  have  not,  or 
by  exposing  thin-leaved 
plants  and  thick  leath- 
ery-leaved ones  to  a  very  dry  atmosphere  without  watering. 
Fig.  121,  however,  may  convey  some  notion  of  the  difference 
between  the  two  kinds  of  structure. 

In  A  the  shaded  part  is  all  cutinized;  it  consists  of  the 
thick  cuticle  proper  and,  beneath  this,  cutinized  layers  of  cell 
wall,  under  which  is  a  heavy  layer  of  cellulose.  In  B  the  cuticle 
is  thin,  and  the  outer  portion  of  the  cell  walls  consists  wholly 
of  cellulose. 

In  most  cases,  as   in   the   india-rubber   tree,  the    external 
epidermal  cells  (and  often  two  or  three  layers  of  cells  beneath 
these)  are  filled  with  water,  and  thus  serve  as  reservoirs  from 
1  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  I,  pp.  273-362. 


FIG.  121.   Unequal  development  of  cuticle 
by  epidermis  cells 

A,  epidermis  of  butcher's  broom  (Ruscus) ;  J5, 
epidermis  of  sunflower ;  c,  cuticle ;  e,  epider- 
mis cells.  —  After  Frank  and  Tschirch 


118          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

which  the  outer  parts  of  the  leaf  and  the  stem  are  at  times 
supplied. 

In  many  cases,  noticeably  in  the  cabbage,  the  epidermis  is 
covered  with  a  waxy  coating,  which  doubtless  increases  the 
power  of  the  leaf  to  retain  needed  moisture,  and  which  certainly 
prevents  rain  or  dew  from  covering  the  leaf  surfaces,  especially 
the  lower  surfaces,  so  as  to  hinder  the  operation  of  the  stornata. 
Many  common  plants,  like  the  meadow  rue  and  the  nasturtium, 
possess  this  power  to  shed  water  to  such  a  degree  that  the  under 
surface  of  the  leaf  is  hardly  wet  at  all  when  immersed  in  water. 
The  air  bubbles  on  such  leaves  give  them  a  silvery  appearance 
when  held  under  water. 

141.  Operation  of  the  stomata.    The  stomata  serve  to  admit 
air  to  the  interior  of  the  leaf  and  to  allow  moisture  in  the  form 
of  vapor  to  pass  out  of  it.    They  do  this  not  in  a  passive  way, 
as  so  many  mere  holes  in  the  epidermis  might,  but  to  a  con- 
siderable extent  they  regulate  the  rapidity  of  transpiration,  open- 
ing more  widely  in  damp  weather  and  in  sunlight,  and  closing 
in  very  dry  weather.    The  opening  is  caused  by  each  of  the 
guard  cells  bending  into  a  more  kidney-like  form  than  usual, 
and  the  closing  by  a  diminution  of  twrgor  and  straightening  out 
of  the  guard  cells. 

The  details  of  the  mechanical  explanation  of  stoma  move- 
ments are  complicated,  and  it  is  difficult  fully  to  account  for 
their  irritability  in  response  to  light,  heat,  and  moisture  stimuli, 
and  to  variations  in  the  amount  of  salts  in  the  water  absorbed 
by  the  roots. 

142.  Location  of  the  stomata.    The  under  side  of  the  leaf, 
free  from  palisade  cells,  abounding  in  intercellular  spaces,  and 
pretty  well  protected  from  becoming  covered  with  rain  or  dew, 
is  especially  adapted  for  the  working  of  the  stomata,  and  accord- 
ingly we  usually  find  them  in  much  greater  numbers  on  the 
lower   surface.    On   the   other   hand,  stomata   occur   only  on 
the  upper  surface  of  the  leaves  of  pond  lilies,  which  lie  flat 
on  the  water.    In  those  leaves  which  stand  with  their  edges 


HAIRS  ON  LEAVES  119 

nearly  vertical,  the  stomata  are  distributed  somewhat  equally 
on  both  surfaces.  Stomata  occur  in  the  epidermis  of  young 
stems,  being  replaced  later  by  the  lenticels. 

The  health  of  the  plant  depends  largely  on  the  proper  work- 
ing condition  of  the  stomata,  and  one  reason  why  plants  in  cities 
often  fail  to  thrive  is  that  the  stomata  become  choked  with  dust 
and  soot.  If  the  stomata  were  to  become  filled  with  water,  their 
activity  would  cease  until  they  were  freed  from  it ;  hence  many 
plants  have  their  leaves,  especially  the  under  surface,  protected 
by  a  coating  of  wax  which  sheds  water. 

143.  Hairs  on  leaves.    Many  kinds  of  leaves  are  more  or  less 
hairy  or  downy,  as  those  of  the  mullein,  the  "  mullein  pink," 
many  cinquefoils,  and  other  common  plants.    In  some  instances 
this  hairiness  may  be  a  protection  against  snails  or  other  small 
leaf-eating  animals,  but  in  other  cases  it  seems  to  be  pretty  clear 
that  the  woolliness  (so  often  confined  to  the  under  surface)  is 
to  lessen  the  loss  of  water  through  the  stomata.    The  Labrador 
tea  is  an  excellent  example  of  a  plant  with  a  densely  woolly 
coating  on  the  lower  surface  of  the  leaf.    The  leaves,  too,  are 
partly  rolled  up  like  those  of  the  crowberry  (Fig.  361),  but  less 
completely,  with  the  upper  surface  outward,  so  as  to  give  the 
lower  surface  a  sort  of  deeply  grooved  form,  and  on  the  lower 
surface  all  of  the  stomata  are  placed.    This  plant,  like  some 
others  with  the  same  characteristics,  ranges  far  north  into  regions 
where  the  temperature,  even  during  summer,  often  falls  so  low 
that  absorption  of  water  by  the  roots  ceases,  since  it  has  been 
shown  that  this  nearly  stops  a  little  above  the  freezing  point  of 
water.    Exposed  to  cold,  dry  winds,  the  plant  would  then  often 
be  killed  by  complete  drying  if  it  were  not  for  the  protection 
afforded  by  the  woolly,  channeled  under  surfaces  of  the  leaves. 

144.  Total  amount  of  transpiration.    In   order  to  prevent 
wilting,  the  rise  of  sap  during  the  life  of  the  leaf  must  have 
kept  pace  with  the  evaporation   from  its  surface.    The  total 
amount  of  water  that  travels  through  the  roots,  stems,  and 
leaves  of  most  seed  plants  during  their  lifetime  is  large,  relative 


120         STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

to  the  weight  of  the  plant  itself.  During  173  days  of  growth  a 
corn  plant  has  been  found  to  give  off  nearly  3 1  pounds  of  water. 
During  140  days  of  growth  a  sunflower  plant  gave  off  about 
145  pounds.  A  grass  plant  has  been  found  to  give  off  its  own 
weight  of  water  every  twenty-four  hours  in  hot,  dry  summer 
weather.  This  would  make  about  6^  tons  per  acre  every  twenty- 
four  hours  for  an  ordinary  grass  field,  or  rather  over  2200  pounds 
of  water  from  a  field  50  X  150  feet,  —  that  is,  from  a  tract  not 
larger  than  a  good-sized  city  lot.  Calculations  based  on  obser- 
vations made  by  the  Austrian  forest  experiment  stations  showed 
that  a  birch  tree  with  200,000  leaves,  standing  in  open  ground, 
transpired  on  hot  summer  days  from  700  to  900  pounds,  while 
at  other  times  the  amount  of  transpiration  was  probably  not 
more  than  18  to  20  pounds.1 

145.  Accumulation  of  mineral  matter  in  the  leaf.  Just  as  a 
deposit  of  salt  is  found  in  the  bottom  of  a  seaside  pool  of  salt 
water  which  has  been  dried  up  by  the  sun,  so  old  leaves  are 
found  to  be  loaded  with  mineral  matter  left  behind  as  the  sap 
drawn  up  from  the  roots  is  evaporated  through  the  stomata. 
A  bonfire  of  leaves  makes  a  surprisingly  large  heap  of  ashes.    An 
abundant  constituent  of  the  ashes  of  burnt  leaves  is  silica,  a 
substance  chemically  the  same  as  sand.    This  the  plant  is  forced 
to  absorb  along  with  the  potash,  compounds  of  phosphorus,  and 
other  useful  substances  contained  in  the  soil  water ;  but  since 
the  silica  is  of  hardly  any  value  to  most  plants,  it  often  accumu- 
lates in  the  leaf  as  so  much  refuse.    Lime  is  much  more  useful 
to  the  plant  than  silica,  but  a  far  larger  quantity  of  it  is  absorbed 
than  is  needed ;  hence  it,  too,  accumulates  in  the  leaf. 

146.  The  fall  of  the  leaf.    In  the  tropics  trees  retain  most  of 
their  leaves  the  year  round  ;  a  leaf  occasionally  falls,  but  no  con- 
siderable portion  of  them  drops  at  any  one  season.2    The  same 

1  See  B.  E.  Fernow's  discussion  in  Report  of  Division  of  Forestry  of  United 
States  Department  of  Agriculture,  1880  ;  also  the  article,  "  Water  as  a  Factor 
in  the  Growth  of  Plants,"  by  B.  T.  Galloway  and  Albert  F.  Woods,  Year- 
Book  of  United  States  Department  of  Agriculture,  1894. 

2  Except  where  there  is  a  severe  dry  season. 


THE  FALL  OF  THE  LEAF  121 

statement  holds  true  in  regard  to  our  cone-bearing  evergreen 
trees,  such  as  pines,  spruces,  and  the  like.  But  the  impossibility 
of  absorbing  soil  water  when  the  ground  is  at  or  near  the  freez- 
ing temperature  would  cause  the  death,  by  drying  up,  of  trees 
with  broad  leaf  surfaces  in  a  northern  winter.  And  in  countries 
where  there  is  much  snowfall,  most  broad-leafed  trees  could 
only  escape  injury  to  their  branches  from  overloading  with  snow, 
by  encountering  winter  storms  in  as  close-reefed  a  condition  as 
possible.  For  such  reasons  our  common  shrubs  and  forest  trees 
(except  the  cone-bearing,  narrow-leafed  ones  already  mentioned) 
are  mostly  deciduous,  —  that  is,  they  shed  their  leaves  at  the 
approach  of  winter.  There  are,  however,  in  the  eastern  United 
States  a  few  species  of  broad-leafed  evergreen  trees  and  large 
shrubs,  such  as  the  live  oak,  some  Rhododendrons,  the  mountain 
laurel  (Kalmia),  and  the  hollies.  Along  the  Pacific  coast  there 
are  many  more  forms,  including  five  fairly  common  species  of 
evergreen  oaks,  the  beautiful  Arbutus,  and  the  manzanitas 
(Arctostaphylos). 

Looking  somewhat  closely  into  the  matter  of  deciduousness 
of  the  trees  and  shrubs  of  temperate  climates  (not  including  the 
coniferous  species),  one  finds  that  they  may  be  classed  as  follows  : 

I.  Leaves  simultaneously  deciduous  .     .\*>  winter  deciduous 

j  B,  summer  deciduous 

C,  leaves  some  of  them 
lasting  two  years  or 


II.  Leaves  not  simultaneously  deciduous 


(evergreen) 


more 


D,  leaves  lasting  more 
than  one  year  but  less 
than  two 

The  only  one  of  the  four  subdivisions  which  shows  fairly  con- 
stant leafage  at  all  seasons  is  the  one  designated  as  C.  Leaves 
of  the  subdivision  D  often  fall  when  about  fifteen  months  old, 
so  that  the  tree  is  unusually  leafy  during  the  three  months 
when  the  new  leaves  are  developing  to  their  full  size,  but  before 
the  old  ones  begin  to  fall.  It  is  a  noteworthy  fact  that  in  many 
species  of  broad-leafed  evergreens,  for  example  the  ilex  oak,  the 


122          STRUCTURE  AND  FUNCTIONS  OF  LEAVES 

oleander,  and  Smilax  aspera,  the  leaves  do  not  attain  their  maxi- 
mum power  of  transpiration  as  soon  as  they  are  fully  grown. 
Such  a  leaf  transpires  more  when  fifteen  months  old  than  when 
three  months  old. 

The  fall  of  the  leaf  is  preceded  by  important  changes  in  the 
contents  of  its  cells.  Much  of  the  sugary  and  protoplasmic  con- 
tents of  the  leaf  disappears  before  it  falls.  These  valuable  mate- 
rials have  been  absorbed  by  the  branches  and  roots,  to  be  used 
again  the  following  spring. 

The  separation  of  the  leaf  from  the  twig  is  accomplished  by 
the  formation  of  a  layer  of  cork  cells  across  the  base  of  the 
petiole  in  such  a  way  that  the  latter  finally  breaks  off  across 
the  surface  of  the  layer.  A  waterproof  scar  is  thus  already 
formed  before  the  removal  of  the  leaf,  and  there  is  no  waste  of 
sap  dripping  from  the  wound  where  the  leafstalk  has  been 
removed,  and  no  chance  for  fungi  to  attack  the  bark  or  wood 
and  cause  it  to  decay.  In  compound  leaves  each  leaflet  may 
become  separated  from  the  petiole,  as  is  notably  the  case  with 
the  horse-chestnut  leaf  (Fig.  99).  In  woody  monocotyledons, 
such  as  palms,  the  leafstalks  do  not  commonly  break  squarely 
off  at  the  base,  but  wither  and  leave  projecting  stumps  on 
the  stem. 

The  brilliant  coloration,  yellow,  scarlet,  deep  red,  and  purple, 
of  autumn  leaves  is  popularly  but  wrongly  supposed  to  be  due 
to  the  action  of  frost.  "It  depends  merely  on  the  changes  in  the 
chlorophyll  grains  and  the  liquid  cell  contents  that  accompany 
the  withdrawal  of  the  proteid  material  from  the  tissues  of  the 
leaf.  The  chlorophyll  turns  into  a  yellow,  insoluble  substance 
after  the  valuable  materials  which  accompany  it  have  been 
taken  away,  and  the  cell  sap  at  the  same  time  may  turn  red. 
Frost  perhaps  hastens  the  break-up  of  the  chlorophyll,  but  indi- 
vidual trees  often  show  bright  colors  long  before  the  first  frost, 
and  in  very  warm  autumns  most  of  the  changes  in  the  foliage 
may  come  about  before  there  has  been  any  frost. 


CHAPTEE  XIII 
THE  FLOWER  OF  THE  HIGHER  SEED  PLANTS 

147.  Organs  of  the  flower.  The  parts  found  in  a  complete 
flower  of  the  higher  seed  plants  (angiosperms)  are  sepals,  petals, 
stamens,  and  pistils  (Fig.  122).  The  sepals,  taken  together,  con- 
stitute the  calyx ;  the  petals,  taken  together,  constitute  the 


pe 


FIG.  122.   Face  view  and  dissection  of  an  angiospermous  flower 
r,  receptacle;  .96,  sepal;  pe,  petal;  st,  stamen;  pi,  pistil;  o,  ovule 

corolla.  The  calyx  and  corolla  are  collectively  known  as  the 
floral  envelopes,  or  perianth. 

Many  angiospermous  flowers  consist  of  five  circles,  or  whorls, 
two  of  which  belong  to  the  perianth,  two  to  the  stamens,  and  one 
to  the  pistils.  The  parts  of  each  circle  alternate  in  position  with 
those  of  the  preceding  or  following  one,  and  all  the  members  of 
each  circle  are  alike  (Fig.  122). 

148.  Suppression  and  multiplication  of  circle.  Any  circle,  or 
part  of  a  circle,  may  be  suppressed.  If  one  set  of  parts  of  the 
perianth  is  lacking  it  is  assumed  to  be  the  corolla  (Fig.  123).1 

1  For  other  instances  of  suppression  of  various  sets,  see  Bergen,  Flora 
of  the  Northern  States  (Figs.  3,  8,  9, 11,  16). 

123 


124 


FLOWER  OF  THE  HIGHER  SEED  PLANTS 


A  whorl  of  stamens  is  frequently  suppressed,  so  that  only  one 
circle  is  present  in  the  flower  (Fig.  128). 

Multiplication  of  whorls  is  particularly  frequent  among  the 
stamens,  but  other  whorls  may  also  show  it  (see  Figs.  149,  150). 


FIG.  123.  Flower  of 
(European)  wild 
ginger,with  calyx 
but  no  petals 

After  Wossidlo 


A  B 

FIG.  124.   Flowers  of  willow 

A,  staminate  flower;  B,  pistil- 
late flower.  Magnified.  — After 
Decaisiie 


149.  Unisexual  flowers.  The  stamens  and  pistils  may  be 
produced  in  separate  flowers,  which  are  unisexual  (often  called 
imperfect)  flowers.  In  the  very  simple  unisexual  flowers  of  the 
willow  (Fig.  124)  each  flower  of  the  catkin  (Fig.  143)  consists 
merely  of  a  pistil  or  a  group  of  (usually  two)  stamens  springing 
from  the  axil  of  a  small  bract. 

Staminate  and  pistillate  flowers  may  be 
borne  on  different  plants,  as  they  are  in 
the  willow,  or  they  may  be  borne  on  the 
same  plant,  as  in  the  hickory  and  the  hazel 
among  trees,  or  in  the  castor-oil  plant, 
Indian  corn,  and  the  begonias.  When  stam- 
inate and  pistillate  flowers  are  borne  on 
separate  plants,  such  a  plant  is  said  to  be 
dioecious,  that  is  of  two  households ;  when 
both  kinds  of  flower  appear  on  the  same 
individual,  the  plant  is  said  to  be  moncecious,  that  is  of  one 
household. 


FIG.  125.  Bilaterally 
symmetrical  flower 
of  pansy 


SYMMETRY  OF  THE   FLOWER 


125 


150.  Symmetry  of  the  flower.  Most  angiosperms  have  sym- 
metrical flowers.  The  simplest  are  those  whose  parts  are  ar- 
ranged as  iii  Figs.  122,  128,  and  149,  having  radial  symmetry.1 


FIG.  126.   Bilaterally  symmetrical  flower  of  sweet  pea 
A,  side  view;  B,  front  view,  dissected;  s,  standard;  w,  w,  wings;  k,  keel 

A  higher  type  of  flower  is  that  which  shows  bilateral  symmetry? 
as  in  Figs.  125  and  126. 

If  the  drawing  of  such  a  flower  were  folded  along  the  axis  of 
symmetry,  one  half  of  the  drawing  would  cover  and  correspond 
with  the  other  half.  Some  flowers  are  wholly 
irregular,  showing  no  sort  of  symmetry. 

151.  The  receptacle.  The  parts  of  the  flower 
are  borne  on  a  variously  formed  expansion 
of  the  flower  stalk  known  as  the  recepta- 
cle. Usually,  as  in  Fig.  122,  this  is  only  a 
slight  enlargement  of  the  flower  stalk,  but 
in  the  rose  (Fig.  127),  the  pond  lily  (Fig.  137), 
the  magnolia,  the  Calycanthus,  and  a  good 
many  other  familiar  flowers  it  is  large  and 
conspicuous. 


FIG.  127.   A  rose 

Longitudinal  section 
After  Decaisne 


1  Such  flowers  are  also  called  actinomorphic,  meaning  ray-formed. 

2  These  are  called  zygomorphic  flowers  (from  Greek  words  signifying  yoke 
and/orm).    In  many  floras  these  are  described,  as  irregular  flowers, 


126  FLOWER  OF  THE  HIGHER  SEED  PLANTS 

152.  The  perianth.  In  dicotyledonous  plants  the  sepals,  or 
divisions  of  the  calyx,  are  commonly  green  and  somewhat  leaf- 
like.  The  petals  in  showy  flowers  are  of  many  colors,  ranging 
all  the  way  from  violet  to  red.  Either  whorl  of  the  perianth 
may  be  found  to  have  assumed  some  very  peculiar  form,  to 
carry  out  the  purpose  of  the  flower,  as  is  briefly  explained  in 
Chapter  xxxn. 

Among  the  lower  families  of  angiosperms  the  parts  of  the 
perianth  are  frequently  all  distinct,  as  shown  in  Figs.  122  and 

134.  Among  the  higher  families 
the  members  of  the  perianth  are 

9  '» "—  -^  \s£~_  /    /  ,p    often  borne  upon  a  tubular  or  cup- 

like  outgrowth  from  the  recep- 
tacle (Fig.  136,  B),  so  that  the 
sepals  or  petals,  or  both,  appear 
to  have  grown  together  more  or 
less  completely.1 

When  the  calyx  or  the  corolla 

is   borne  upon   a  tubular,  bowl- 
FIG.  128.   Flower  of  Hydrophyllum     11,1 

shaped,  or  other  extension  of  the 

cal,  lobe  of  calyx ;   cor,  lobe  of  co- 

roiia;   st,  stamens;  p,  pistil,   receptacle,   there   are  often  divi- 
Modified.- After  Lindley  sionSj  teeth,  or  lobes  at  the  rim 

of  the  tube  (Figs.  128,  144,  Appendix)  showing  how  many 
sepals  or  petals  the  flower  possesses.  Special  names  are  given 
to  a  large  number  of  forms  of  the  sympetalous  corolla,  and 
these  are  of  much  use  in  accurate  descriptions  of  seed  plants. 
A  few  of  these  are  illustrated  in  Chapter  xxxn  and  in  the 
Appendix. 

1  When  the  parts  of  the  perianth  are  distinct  the  calyx  is  said  to  be  chori- 
sepalous  and  the  corolla  choripetalous ;  other  terms  are  polysepalous  and 
polypetalous.  When  the  receptacle  forms  a  cup-like  or  tubular  outgrowth 
so  that  the  teeth  or  lobes  of  this  alone  are  sepals  or  petals,  the  flower  is  said 
to  be  synsepalous  or  sympetalous ;  other  terms  are  gamosepalous  orgamopeta- 
lous.  Choris  means  apart,  poly  means  many,  syn  means  together,  gamos 
means  marriage.  Botanists  have  until  recently  used  such  expressions  as 
"sepals  united  into  a  tube,"  etc.,  but  these  are  incorrect. 


FORMS  AND   UNION   OF  STAMENS 


127 


A 


FIG.  129.   Parts  of 
a  stamen 


153.  Form  of  the  stamen  ;  union  of  stamens.  Stamens  are 
of  many  specialized  forms,  to  adapt  them  to  their  functions  in 
flowers  of  various  shapes,  but  many  are  of  the  shape  shown  in 
Fig.  129.  Such  a  stamen  consists  of  an  ex- 
panded part,  the  anther,  borne  on  a  stalk 
called  i\\Q  filament.  Anthers  are  often  nearly 
or  quite  sessile  (seated,  i.e.  destitute  of  fila- 
ments). Inside  the  anther  is  the  powdery  or 
pasty  substance  called  pollen  (Fig.  153). 

Stamens  may  be  wholly  unconnected  with 
each  other,  or  distinct,  as  shown  in  Figs.  122, 
124,  and  128,  or  they  may  be  really  or  ap- 
parently more  or  less  united  to  each  other. 

In  Fig.  130  the  stamens 

have    arisen    separately, 

but  finally  become  joined 

together  by  their  anthers 

(as  is  always  the  case  in 

the    family    Composite). 

In  other  cases  the  stamens  appear  united 

when  they  are  not  really  so,  because  they 

are  borne  on  a  ring  or  tube  of  tissue,  as 

already  explained    in  connection  with  the 

perianth  (Sec.  152). 

Without  regard  to  whether  the  union  is 

real  or  apparent,  stamens  which  occur  in  a 
FIG.  130.  Stamens  of      .      -,  ,,•,      £1  ..... 

a  thistle,  with  an-  smgle  8rouP  (the  filaments  appearing  joined) 
there  united  into  a  are  said  to  be  monadelphous  (Fig.  131),  in 
rinS  two  groups,  diadelphous  (Fig.  132),  in  many 

a,  united  anthers;  /,   groups,  polyadelphous   (the  terms   meaning 

filaments,  bearded    *  ,    * 

on  the  sides.  — After   one  brotherhood,  two  brotherhoods,    many 

Baiiion  brotherhoods). 

154.  The  carpel.  The  simplest  form  of  the  organ  which  bears 
the  structures  called  ovules,  that  are  to  mature  into  seeds,  is 
known  as  the  carpel. 


B, 

c, 

ive ;    /,    filament.  — 
After  Strasburger 


128 


FLOWER  OF  THE  HIGHER  SEED  PLANTS 


In  the  lowest  of  the  two  great  groups  of  seed  plants,  the 
gymnosperms  (meaning  naked  seeds),  to  which  the  pines,  spruces, 
cedars,  and  the  like  belong,  the  ovules  are 
borne  exposed  on  the  surface  of  the  carpels, 
which  usually  have  the  form  of  scales.  But 
in  the  higher  group  of  seed  plants,  the  angio- 
sperms  (meaning  seeds  in  a  vessel),  the  car- 
pels constitute  a  part  of  cases  or  chambers 
in  which  the  ovules  are  formed  and  which 
are  generally  quite  closed. 

FIG.  131.  Monadel-  155-  The  Pistil-  The  term  pistil  (Latin 
phous  stamens  of  for  pestle)  is  applied  to  the  closed  structure 
mallow  which  contains  the 

ovules  and  is  formed  by  the  carpels  of  the 
angiosperms.    This  is  a  more  general  term 
than  carpel,  for  it  applies  to  organs  com- 
posed of  one  or  of  several   FlG- 132'  Diadelphous 
.          .  stamens  of  sweet  pea 

carpels.    If  a  pistil  is  of  one 

carpel  it  is  said  to  be  simple,  if  of  two  or  more  car- 
pels it  is  compound. 

The  pistil  often  consists  of  an  enlarged,  hol- 
low portion  containing  ovules  and  known  as  the 
ovary,1  a  stalk-like  style,  and  a  knob  or  ridged 
expansion  called  the  stigma  (Fig.  133).  Not  infre- 
quently the  style  is  wanting  and  the  stigma  is 
FIG  133  Parts  sess^e  (seated)  on  the  ovary. 

of  the  pistil  A  flower  may  contain  several  or  many  carpels 

ow,  ovary;  *ty,  in  the  form  of  simple  pistils  separate  from  one 

style;  stig,   another,  as  in   the  stonecrop  and  the  buttercup 

(Figs.  134,  161).    When  several  carpels  form  a 

compound  pistil  the  manner  and  extent  of  the  union  is  various. 

1  The  term  ovary  is  an  unfortunate  one,  since  it  would  seem  to  mean  the 
organ  which  bears  eggs.  Those  who  wish  to  avoid  the  use  of  the  term  may 
substitute  the  word  ovulary,  proposed  by  Professor  Charles  R.  Barnes,  or 
may  simply  say  ovule  case. 


stig—GS 


sty---- 


SIMPLE  AND  COMPOUND  PISTILS 


129 


The  union  generally  forms  the  ovary,  although  this  is  sometimes 
developed  in  large  part  as  a  cup-like  or  tubular  growth  from  the 
stem  under  the  carpels.  Sometimes  the  union  is  so  complete 


FIG.  134.  Flower  of  stonecrop 
A,  entire  flower;   Z>,  vertical  section.  — After  Decaisne 

that  the  compound  pistil  has  only  one  style  and  one  stigma ;  but 
frequently  the  styles  remain  separate,  or  the  styles  may  be 
united  and  the  stigmas  separate,  or  at  least  lobed  so  as  to  show 
of  how  many  carpels  the  pistil  is  made  up  (Figs.  123,  124). 
Even  when  there  is  no  external  sign  to  indicate  the'  compound 
nature  of  the  pistil,  it  can  usually  be  recognized  from  a  study  of 
a  cross  section  of  the  ovary. 

156.  Locules  of  the  ovary;  placentas.  Compound  ovaries 
very  commonly  consist  of  a  number  of  separate  chambers  known 
as  locules.1  Fig.  135,  B,  shows 
a  three-loculed  ovary  seen  in 
cross  section.  The  ovules  are 
not  borne  indiscriminately  by 
any  part  of  the  lining  of  the 
ovary.  In  one-loculed  pistils 
they  frequently  grow  in  a  line 
running  along  one  side  of  the 
ovary,  as  in  the  pea  pod  (Fig. 
343).  The  ovule-bearing  line  is 


C 


FIG.  135.   Principal  types  of  placenta 

A,  parietal  placenta  ;  B,  central  placenta  ; 
C,  free  central  placenta  ;  A  and  B,  trans- 
verse sections;  C,  longitudinal  section. 
—  After  Strasburger 


called  a  placenta ;  in  compound  pistils  there  are  commonly  as 
many  placentas  as  there  are  separate  carpels  joined  to  make  the 


Often  (less  correctly)  called  cells. 


130 


FLOWER  OF  THE  HIGHER  SEED  PLANTS 


pistil.    Placentas  on  the  wall  of  the  ovary,  like  those  in  Fig.  13  5, 
A,  are  called  parietal  placentas  ;  those  which  occur  as  at  B 

are  said  to  be  axial;  and  those 
which,  like  the  form  repre- 
sented in  C,  consist  of  a  col- 
umn rising  from  the  bottom  of 
the  ovary  are  called  free  cen- 
tral placentas. 
157.  Superior,  half  -inferior, 

anfl 


FIG.  136.   Insertion  of  the  floral  organs 

A,  hypogynous,  all  the  other  parts  on  the  as  in  the  diagrammatic  flower 

receptacle,  beneath  the  pistil  ;  P>,  perig-  .                 & 

ynous,  petals  and  stamens  apparently  of  Fig.  122,  the   receptacle    IS 

growing  out  of  the  calyx,  around  the  rounded   or   club-shaped,  and 
pistil  ;  C,  cpigynous,  all  the  other  parts 

appearing  to  grow  out  of  the  pistil.—  the  floral  organs  arise  from  it 


After  Strasburger 


in  successive  gets> 


flower 


is  said  to  be  liypogynous  (from  two  Greek  words  here  applied 
to  mean  under  the  pistil),  and  the  ovaries  are  said  to  be 
superior  (Fig.  136,  A). 

When  the  receptacle  is  concave,  or  when  it  grows  up  about 
the  pistil,  carrying  the  other  floral  parts  with  it,  so  that 
the  pistil  is  inserted  on  the  same 
level  with  the  stamens  or  lower, 
but  not  at  all  united  with  the  re- 
ceptacle, the  flower  is  said  to  be 
perigynous  (meaning  around  the 
pistil)  and  the  ovary  is  half  infe- 
rior (Fig.  136,  B). 

When  the  ovary  is  united  with 

the  receptacle  the  flower  is  said 

v  .  . 

to   be  epigynous   (meaning   upon 


FIG.  137.   White  water  lily 


,i          .,.-,.  ,,  .     „      The  inner  petals  and  the  stamens 

the   pistil),    Or    the    OVary  IS    infe-       growing  from  the  ovary.  -  After 
rior  (Fig.  136,  C}.  Decaisne 

158.  Floral  diagrams.  Sections  (real  or  imaginary)  through 
the  flower  lengthwise,  like  those  of  Fig.  136,  help  greatly  in 
giving  an  accurate  idea  of  the  relative  position  of  the  floral 


FLORAL   DIAGRAMS 


131 


organs.  Equally  important  in  this  way  are  cross  sections, 
which  may  be  recorded  in  diagrams  like  those  of  Fig.  138.1 
In  constructing  such  diagrams  it  will  often  be  necessary  to 
suppose  some  of  the  parts  of  the  flower  to  be  raised  or  lowered 
from  their  true  position,  so  as  to  bring  them  into  such  rela- 

tions  that  all  could  be  cut  by  a  single 
section.  This  would,  for  instance,  be 
necessary  in  making  a  diagram  for  the 


A  B  C 

FIG.  138.  Diagram  of  cross  sections  of  flowers 

A,  columbine;  B,  heath  family;  C,  Iris  family.  In  each  diagram  the  dot  along- 
side the  main  portion  indicates  a  cross  section  of  the  stem  of  the  plant.  In  B 
every  other  stamen  is  more  lightly  shaded,  because  some  plants  of  the  heath 
family  have  five  and  some  ten  stamens.  —  After  Sachs 

cross  section  of  the  flower  of  the  white  water  lily,  of  which  a 
partial  view  of  one  side  is  shown  in  Fig.  137. 

It  is  found  convenient,  in  diagrams  of  cross  sections,  to  dis- 
tinguish the  sepals  from  the  petals  by  representing  the  former 
with  midribs.  The  diagrammatic  symbol  for  a  stamen  stands 
for  a  cross  section  of  the  anther,  and  that  for  the  pistil  is  a 
section  of  the  ovary.  If  any  part  is  lacking  in  the  flower  (as 
in  the  case  of  flowers  which  have  some  antherless  filaments), 
the  missing  or  abortive  organ  may  be  indicated  by  a  dot.  In 
the  diagram  of  the  Iris  family  (Fig.  138,  C)  the  three  dots 
inside  the  flower  indicate  the  position  of  a  second  circle  of 
stamens,  found  in  most  flowers  of  monocotyledons  but  not  in 
this  family. 

1  For  floral  diagrams  see  Le  Maout  and  Decaisne,  Traite  General  de 
Botanique,  or  Eichler,  Bluthendiagramme. 


CHAPTER  XIV 


INFLORESCENCE 

159.  Definition  of  inflorescence  and  flower  cluster.  The  man- 
ner in  which  flowers  are  arranged  on  the  stem  is  known  as 
inflorescence.1    Not  infrequently  the  flowering  shoot  bears  only 
a  single  flower,  but  very  generally  among  seed  plants  these 
shoots  are  grouped  into  definite  systems,  which   are  called 
flower  clusters. 

160.  Advantage  of  grouping  flowers.    Flowers  when  clus- 
tered, as  in  Figs.  140—143,  on  special  nearly  leafless  shoots  are 
much  more  conspicuous  than  they  would  be  if  scattered  along 

ordinary  leafy  branches  and  partly  hidden 
by  the  leaves.  This  is  a  decided  advantage 
in  securing  many  visits  from  insects  which 
carry  pollen  from  plant  to  plant  (Chapter 
xxxn)  and  leads  to  a  more  abundant  pro- 
duction of  seed. 

161.  Regular  positions  for  flower  buds. 
Flower  buds,  like  leaf  buds,  occur  regularly 
either  in  the  axils  of  leaves  or  at  the  end 
of  the  stem  or  branch,  and  are  therefore 
either  axillary  or  terminal  (Sec.  168). 

162.  Axillary  and  solitary  flowers  ;  inde- 
terminate inflorescence.    The  simplest  pos- 
sible arrangement  for  flowers  which  arise 
from  the  axils  of  leaves  is  to  have  a  single 

flower  spring  from  each  leaf  axil.    Fig.  139  shows  how  this 
plan  appears  in  a  plant  with  opposite  leaves.    As  long  as  the 


FIG.  139.  Axillary 
and  solitary  flowers 
of  pimpernel 


1  Sometimes  (but 
flower  cluster. 


correctly)  the  word  inflorescence  is  used  to  mean 
132 


THE  RACEME  AXD  RELATED  FORMS 


133 


FIG.  140.  Raceme  of 
common  red  currant 

p,  peduncle ;  p',  pedicel 


br,  bract 


stem  continues  to  grow  the  production  of  new  leaves  may  be 
followed  by  that  of  new  flowers.  Since  there  is  no  definite 
limit  to  the  number  of  flowers  which  may  appear  in  this  way, 
the  mode  of  flowering  just  de- 
scribed (with  many  others  of 
the  same  general  character)  is 
known  as  indeterminate  inflo- 
rescence. 

163.  The  raceme  and  re- 
lated forms.     If    the    leaves 
along  the  stem  were  to  become 

very  much  dwarfed  and  the  flowers  brought  closer  together, 
as  they  frequently  are,  a  kind  of  flower  cluster  like  that  of 
the  currant  (Fig.  140)  or  the  lily  of  the  valley  would  result. 
Such  an  inflorescence  is  called  a  raceme ;  the  main  flower 
stalk  is  known  as  the  peduncle ;  the  little  individual  flower 

stalks  are  pedicels,  and  the  small, 
more  or  less  scale-like  leaves  of 
the  peduncle  are  bracts. 

Frequently  the  lower  pedicels 
of  a  cluster  on  the  general  plan  of 
the  raceme  are  longer  than  the 
upper  ones  and  make  a  somewhat 
flat-topped  cluster,  like  that  of  the 
hawthorn,  the  elder,  the  sheep  lau- 
rel, or  the  trumpet  creeper.  This 
is  called  a  corymb. 

In  many  cases,  for  example  the 
parsnip,  the  sweet  cicely,  the  gin- 
seng, and  the  cherry,  a  group  of  pedicels  of  nearly  equal 
length  spring  from  about  the  same  point.  This  produces  a 
flower  cluster  called  the  umbel  (Fig.  141). 

164.  Sessile  flowers  and  flower  clusters.    Often  the  pedicels 
are  wanting,  or  the  flowers  are  sessile,  and  then  a  modification 
of  the  raceme  is  produced  which  is  called  a  spike,  like  that  of 


FIG.  141.    Simple  umbel 
of  cherry 


134 


INFLORESCENCE 


the  plantain  (Fig.  142).  The  willow,  alder,  birch, 
poplar,  and  many  other  common  trees  bear  a  short, 
flexible,  rather  scaly  spike  (Fig.  143),  which  is  called 
a  catkin. 

The  axis  of  the  inflorescence  is  often  so  much  short- 
ened as  to  bring  the  flowers  into  a  somewhat  globular 
mass.    This  is  called  a  head  (Fig.  142).    Around  the 
base  of  the  head  usually  occurs 
a  circle  of  bracts  known  as  the 
involucre.    The  same  name  is 
given  to  a  set  of  bracts  which 
often  surround  the  bases  of  the 
pedicels  in  an  umbel. 

165.  The  composite  head.  The 
plants  of  one  large  group  —  of 
which  the  dandelion,  the  daisy, 
the  thistle,  and  the  sunflower 
are  well-known  members  —  bear 
their  flowers  in  close  involucrate  heads  on  a  common  recep- 
tacle. The  whole  cluster  looks  so  much  like  a  single  flower 
that  it  is  usually  taken  for 
one  by  non-botanical  people. 
In  many  of  the  largest  and 
most  showy  heads,  like  that 
of  the  sunflower  and  the 
daisy,  there  are  two  kinds 
of  flowers, — the  ray  flowers, 
around  the  margin,  and  the 
tubular  disk  flowers  of  the  in- 
terior of  the  head  (Fig.  144). 


FIG.  142.   Spike  of  plantain  and 
he\id  of  red  clover 


FIG.  143.   Catkins  of  willow 
A,  staminate  flowers ;  B,  pistillate  flowers 


The  early  botanists  supposed 

the  whole  flower  cluster  to 

be  a  single  compound  flower. 

This  belief  gave  rise  to  the  name  of  one  family  of  plants, 

Compositce,  —  that  is,  plants  with  compound  flowers.    In  such 


THE   COMPOSITE   HEAD 


135 


ch 


FIG.  144.    Head  of  yarrow 

A,  top  view  (magnified);  B,  lengthwise  section  (magnified);  re,  receptacle;  i, 
involucre ;  r,  ray  flowers ;  d,  disk  flowers ;  c,  corolla ;  s,  stigma ;  ch,  chaff,  or 
bracts  of  receptacle 


FIG.  145 
Panicle  of  oat 


FIG.  146.   Compound  umbel 
of  carrot 


heads  as  those  of  the  tansy,  the  thistle,  the  cudweed,  and  the 
everlasting,  there  are  no  ray  flowers,  and  in  others,  like  those  of 
the  dandelion  and  the  chicory,  all  the  flowers  are  ray  flowers. 


136 


INFLORESCENCE 


166.  Compound  flower  clusters.    If  the  pedicels  of  a  raceme 
branch,  they  may  produce  a  compound  raceme,  or  panicle,  like 
that  of  the  oat  (Fig.  145).1    Other  forms  of  compound  racemes 
have  received  other  names. 

An  umbel  may  become  compound  by  the  branching  of  its 
flower  stalks  (Fig.  146),  each  of  which  then  bears  a  little  umbel, 
called  an  iimbellet. 

167.  Inflorescence  diagrams.    The  plan  of  inflorescence  may 
readily  be  indicated  by  diagrams  like  those  of  Fig.  147. 

p 
P 
f 

f 

I 

/ 

A  BCD 

FIG.  147.   Diagrams  of  inflorescence 
A,  panicle;  B,  raceme;  C,  spike;  D,  head;  E,  umbel 

168.  Terminal  flowers ;  determinate  inflorescence.    The  ter- 
minal bud  of  a  stem  may  be  a  flower  bud.    In  this  case  the 
direct  growth   of  the   stem  is  stopped  or  determined  by  the 
appearance  of  the  flower ;  hence  such  plants  are  said  to  have  a 
determinate  inflorescence.    The    simplest  possible   case  of  this 
kind  is  that  in  which  the  stem  bears  but  one  flower  at  its 
summit. 

169.  The   cyme.    Very    often    flowers   appear    from  lateral 
(axillary)  buds,  below  the  terminal  flower,  and  thus  give  rise  to 
a  flower  cluster  called  a  cyme.    This  may  have  only  three  flowers, 
and  in  that  case  would  look  very  much  like  a  three-flowered 

1  Panicles  may  also  be  formed  by  compound  cymes  (see  Sec.  169). 


THE   CYME  137 

umbel.  But  in  the  indeterminate  inflorescence,  such  as  the 
raceme,  corymb,  and  umbel,  the  order  of  flowering  is  from  below 
upward,  or  from  the  outside  of  the  cluster  inward,  because  the 
lowest  or  the  outermost  flowers  are  the  oldest,  while  in  deter- 
minate forms  of  inflorescence  the  central  flower  is  the  oldest, 


FIG.  148.   Compound  cyme  of  mouse-ear  chickweed 
t,  the  terminal  (oldest)  flower 

and  therefore  the  order  of  blossoming  is  from  the  center  out- 
wards. Cymes  are  very  commonly  compound,  like  those  of 
the  elder  and  of  many  plants  of  the  pink  family,  such  as  the 
sweet  william  and  the  mouse-ear  chickweed  (Fig.  148).  They 
may  also,  as  already  mentioned,  be  panicled,  thus  making  a 
cluster  much  like  Fig.  147,  A. 


CHAPTER  XV 

ORIGIN  AND  STRUCTURE  OF  FLORAL  ORGANS ;  POLLINATION 
AND  FERTILIZATION 

170.  The  flower  a  shortened  and  greatly  modified  branch. 

In  Chapter  IX  the  leaf  bud  was  explained  as  being  an  unde- 
veloped branch,  which  in  its  growth  would  develop  into  a  real 
branch  (or  a  prolongation  of  the  main  stem).  Now,  since  flower 
buds  appear  regularly  either  in  the  axils  of  leaves  or  as  terminal 


FIG.  149.   Transition  from  bracts  to  sepals  in  a  cactus  flower 

buds,  there  is  reason  to  regard  them  as  of  a  nature  similar  to 
leaf  buds.  This  would  imply  that  the  receptacle  corresponds  to 
the  axis  of  the  buds  shown  in  Fig.  85,  and  that  at  least  some 
of  the  parts  of  the  flower-  correspond  to  leaves.  There  is  plenty 
of  evidence  that  this  is  really  true.  Sepals  frequently  look  very 
much  like  leaves,  and  in  many  cactuses  the  bracts  about  the 
flower  are  so  sepal-like  that  it  is  impossible  to  tell  where  the 
bracts  end  and  the  sepals  begin  (Fig.  149).  The  same  thing  is 
true  of  sepals  and  petals  in  such  flowers  as  the  white  water  lily. 
In  this  flower  there  is  also  a  remarkable  series,  ranging  all  the 

138 


DEVELOPMENT  OF   THE  ANTHER  139 

way  from  petals  tipped  with  a  bit  of  anther,  through  stamens 
with  a  broad  petal-like  filament,  to  regular  stamens,  as  is  shown 
in  Fig.  150,  A,  B,  C,  D.  The  same  thing  is  shown  in  many  double 
roses.  In  completely  double  flowers  the  stamens  and  pistils  are 
transformed  into  petals  by  cultivation.  In  the  flowers  of  the 
cultivated  double  cherry  the  pistils  occasionally  take  the  form 
of  small  leaves,  and  some  roses  turn  wholly  into  green  leaves. 

Summing  up,  then,  we  know  that  flowers  are    altered   and 
shortened  branches,  (1)  because  flower  buds  have,  as  regards 


BCD 


FIG.  150.   Transitions  from  petals  to  stamens  in  white  water  lily 
A,  B,  C,  D,  various  steps  between  petal  and  stamen.  —  After  Brown 

position,  the  same  kind  of  origin  as  leaf  buds ;  (2)  because  all 
the  intermediate  steps  are  found  between  bracts  on  the  one 
hand  and  petals  on  the  other. 

171.  Development  of  the  anther.    If  the  development  of  an 
anther  is  followed  throughout,  it  will  be  found  at  an  early  stage 
to  contain,  usually,  four  regions,  where  rapid  cell  division  is 
going  on,  which  become  organized  into  pollen  sacs.    These  cavi- 
ties (Fig.  151)  are  filled  with  pollen  grains  and  finally  merge  into 
two  pollen  chambers  which,  in  the  commonest  type  of  anther, 
split  open  lengthwise  to  allow  the  escape  of  the  pollen. 

172.  Relation  of  stamens  and  carpels  to  structures  in  the 
lower  plants.    The  exact  significance  of  the  stamens  and  car- 
pels as  organs  of  the  plant  body  set  apart  for  the  purpose  of 


140     STRUCTURE  OF  FLORAL  ORGANS;  FERTILIZATION 


FIG.  151.    Cross  section  of  anther 
of  mint 


reproduction  can  only  be  understood  by  means  of  a  study  of 
certain  forms   in  the  fern  group,  or  pteridophytes ;  for  these 

structures  had  their  origin  in 
connection  with  the  develop- 
ment,  from  simpler  conditions 
among  the  fern  group,  of  the 
habit  of  producing  seeds.  The 
subject  is  treated  at  some 
length  in  Part  II,  Chapters 
xxvi  to  xxx  inclusive. 
173.  The  anther  and  its 

s,  pollen  sacs,  with  grains  of   pollen;    contents.     Some  of  the  shapes 
d,  groove  along  which  the  anther  will      £          ,1  i         i 

split  open.    Somewhat  magnified.  -  of   anthers    may  be    learned 

After  Bonnier  and  Sablon  from  FigS.  129,  130,  and  152.1 

The  shape  of  the  anther  and  the  way  in  which  it  opens  depend 
largely  upon  the  manner  in  which  the  pollen  is  to  be  discharged 
and  how  it  is  carried  from  flower  to  flower.  The  commonest 
method  is  that  in  which  the 
anther  cells  split  lengthwise,  as 
in  Fig.  152,  A.  A  few  anthers 
open  by  trapdoor-like  valves,  as 
in  B,  and  a  larger  number  by 
little  holes  at  the  top,  as  in  C. 

The  pollen  in  many  plants 
with  inconspicuous  flowers  (as 
the  evergreen  cone-bearing  trees, 
the  grasses,  rushes,  and  sedges) 
is  a  fine,  dry  powder.  In  plants 
with  showy  flowers  it  is  often 
somewhat  sticky  or  pasty.  The 
forms  of  pollen  grains  are  ex- 
tremely various.  Fig.  153  will  serve  to  furnish  examples  of 
some  of  the  shapes  which  the  grains  assume ;  c  in  that  figure 
is  perhaps  as  common  a  form  as  any.  Each  pollen  grain 
1  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  II,  pp.  86-95. 


FIG.  152.   Modes  of  discharging 
pollen 

A,  by  longitudinal  slits  in  the  an- 
ther cells  (amaryllis) ;  B,  by  uplift- 
ing valves  (barberry) ;  (7,  by  a  pore 
at  the  top  of  each  anther  lobe  (night- 
shade) .  —  After  Baillon 


GERMINATION  OF  POLLEN   GRAINS 


141 


consists  mainly  of  a  single  cell,  and  is  covered  by  a  moderately 
thick  outer  wall  and  a  thin  inner  one.  Its  contents  are  thickish 
protoplasm,  full  of  little  opaque  particles  and  usually  containing 


bed 
FIG.  153.  Pollen  grains 

a,  pumpkin;  b,  enchanter's  nightshade;  c,  Albuca ;  d,  pink;  e,  hibiscus.    Very 
greatly  magnified.  —  After  Kerner 

grains  of  starch  and  small  drops  of  oil.  During  the  germination 
of  a  pollen  grain  the  outer  coat  bursts  at  some  point,  forced  out- 
ward by  the  pressure  of  a  tube  formed  from  the  tough  inner  coat. 
Sometimes,  as  in  Fig.  153,  b,  there  are  knobs  or  other  indications 
of  the  places  at  which  the  outer  coat 
is  most  easily  ruptured.  After  the  tube 
has  pushed  its  way  out  it  continues  to 
elongate  rather  rapidly. 

174.  Microscopical  structure  of  the 
stigma  and  style.  Under  a  moderate 
power  of  the  microscope  the  stigma  is 
seen  to  consist  of  cells  set  irregularly 
over  the  surface,  and  secreting  a  moist 
liquid  to  which  the  pollen  grains  ad- 
here (Fig.  154).    Beneath  these  super-  FlG  154    stigma  of  thorn 
ficial  cells  is  spongy  parenchyma,  which      apple  (Datura),  with 
runs  down  through  the  style,  if  there  is      P°llen 
one,  to  the  ovary.    In  some  pistils  the    Magnified.  —  After  Faguet 
pollen  tube  proceeds  through  the  cell  walls,  which  it  softens  by 
means  of  a  substance  which  it  exudes  for  that  purpose.    In  other 
cases  (Fig.  155)  there  is  a  canal  or  passage  along  which  the  pollen 
tube  travels  on  its  way  to  the  ovule. 


142     STRUCTURE  OF  FLORAL  ORGANS;   FERTILIZATION 


175.  Pollination.  The  transference  of  pollen  from  anthers  to 
stigmas  is  called  pollination.  In  the  case  of  plants  with  dry, 
dust-like  pollen  this  is  generally  due  to  the  action  of  the  wind. 
Moist,  sticky  pollen  is  generally  carried  by  some  kind  of  animal, 
usually  by  insects.  The  subject  of  pollination  is  so  important, 

especially  in  relation  to  the  visits  of 
insects,  that  it  needs  a  chapter  by 
itself  (see  Chapter  xxxn). 

176.  Fertilization.  By  fertiliza- 
tion in  seed  plants  the  botanist 
means  the  union  of  a  male  sexual 
nucleus  from  a  pollen  grain  with  the 
female  nucleus  of  the  egg  cell  at  the 
apex  of  the  embryo  sac  (Fig.  157). 
This  process  gives  rise  to  a  cell 
which  contains  protoplasm  derived 
from  the  pollen  tube  and  from  the 
egg  cell.  In  many  plants  the  pol- 
len, in  order  best  to  secure  fertiliza- 
tion, must  come  from  another  plant 
of  the  same  kind,  and  not  from  the 
individual  which  bears  the  ovules  to 
be  fertilized. 

Pollen  tubes  (Fig.  156)  begin  to 
form  soon  after  pollen  grains  lodge  on 
the  stigma.  The  time  required  for 
the  process  to  begin  varies  in  differ- 
ent kinds  of  plants,  requiring  in  many  cases  twenty-four  hours 
or  more.  The  length  of  time  needed  for  the  pollen  tube  to  make 
its  way  through  the  style  to  the  ovary  depends  upon  the  length  of 
the  style  and  other  conditions.  In  the  crocus,  which  has  a  style 
several  inches  long,  the  descent  takes  from  one  to  three  days. 

Finally  the  tube  penetrates  the  opening  at  the  apex  of  the 
ovule  (Fig.  157,  m),  called  the  micropyle  (meaning  little  gate), 
and  transfers  a  male  nucleus  into  the  egg  cell. 


FIG.  155.   Pollen  grains  produ- 
cing tubes,  on  stigma  of  a  lily 

g,  pollen  grains ;  t,  pollen  tubes ; 
p,  papillae  of  stigma;  c,  canal 
or  passage  running  toward 
ovary.  Much  magnified. — 
After  Dodel-Port 


NATURE  OF  THE   FERTILIZING  PROCESS 


143 


177.  Nature  of  the  fertilizing  process.  The  necessary  feature 
of  the  process  of  fertilization  is  the  union  of  the  essential  contents 
of  two  cells,  especially  the  nuclei,  to  form  a  new  one  from  which 
the  future  plant  is  to  spring.  This  kind  of  union  also  occurs  in  all 
the  lower  plants  (Chapters  xx-xxxi),  resulting  in  the  formation  of 
a  spore  capable  of  growing  into  a  complete  plant  like  that  which 


FIG.  156.   Germination  of  the  pollen  grain  of  an  angiosperm 

A,  inner  coat  of  the  pollen  grain  distended  hy  osmosis  from  contact  with  the  moist 
stigma,  and  protruding  slightly  at  the  print  i;  />,  the  pollen  tube  beginning 
to  form ;  C,  the  pollen  tube  more  elongated,  with  the  tube  nucleus  t  at  its  tip, 
the  generative  cell  g  having  begun  to  enter  the  tube ;  D,  the  pollen  tube  still 
farther  elongated ;  E,  the  division  of  the  nucleus  of  the  generative  cell  to  form 
the  two  sperm  nuclei  si  and  $2;  f\  the  sperm  cells  s\  and  ,s2  fully  formed, 
and  the  tube  nucleus  t  breaking  down;  G,  the  tube  nucleus  has  disappeared, 
and  the  sperm  cells  are  about  to  be  discharged  near  the  tip  of  the  pollen  tube. 
Somewhat  diagrammatic  and  much  magnified.  — After  Bonnier  and  Sablon 

produced  it.    It  is  a  sexual  act  and  can  be  studied  much  better  in 
some  of  the  algre,  mosses,  and  ferns  than  in  seed  plants. 

178.  Development  of  the  embryo.  After  fertilization  the 
egg  cell  finally  develops  the  embryo  of  the  future  seed.  This 
formation  of  the  embryo  is  always  a  complicated  process  and 
varies  much  in  different  groups  of  seed  plants.  Briefly  stated, 


144     STRUCTURE  OF  FLORAL  ORGANS ;   FERTILIZATION 


the  process  in  angiosperms  is  as  follows.    The  egg  cell  (Fig. 

158,  A)  some  time  after  fertilization  forms  a  transverse  partition 

and  is  thus  divided  into  two 
cells,  one  of  which  (Fig.  158, 
B,  s)  is  to  form  the  slender 
suspensor  of  the  embryo 
(which  serves  various  pur- 
poses, such  as  forcing  the 
embryo  into  the  nutritive 
tissue  of  the  seed,  absorb- 
ing food  from  the  wall  of  the 
ovary,  or  storing  food  for 
the  growing  embryo)  and  the 
other  (e)  is  to  form  the  embryo 
itself.  These  cells  in  turn 
subdivide,  as  shown  in  C,  D, 
and  E.  The  whole  pear- 
shaped  body  in  parts  B-E  is 
called  the  pro-embryo,  and 
this  continues  to  grow  and 
its  cells  to  subdivide  until 
its  structure  becomes  highly 
complex.  Finally  it  con- 

i,  inner  coating  of  ovule;  o,  outer  coating 

of  ovule ;  p,  pollen  tube  proceeding  from    tains  many  sharply  defined 

one  of  the  pollen  grains  on  the  stigma;  iong  wMdl  gradually  de- 

c,  the  place  where  the  two  coats  of  the 

ovule  blend.    (The  kind  of  ovule  here     velop  into  the  Several  organs 


FIG.  157.   Diagrammatic   representation 
of  fertilization  of  an  ovule 


along  one  side  of  the  ovule.)    a  to  e,  em-          179.    Number    of    pollen 

^ai/SfcrJ^SS  grains  to  each  ovule.    Only 

nucleus  of  the  embryo  sac;  e,  nucleated    one  pollen  grain  is  necessary 

cells,  one  of  which,  the  egg  cell,  receives     ,      »     .  M.  -,  i      i 

the  male  nucleus  of  the  pollen  tube ;  /,  f u-    to  fertilize  each  OVule,  but  SO 

nicuius  or  stalk  of  ovule ;  m,  micropyle  or    many  pollen  grains  are  lost 

opening  into  the  ovule.  —  After  Luerssen       .     ,       , 

that  plants   produce    many 

more  of  them  than  they  do  ovules.    The  ratio,  however,  varies 
greatly.    In  the  night-blooming  cereus  there  are  about  250,000 


NUMBER  OF  POLLEN   GRAINS  TO  EACH  OVULE     145 

pollen  grains  for  30,000  ovules,  or  rather  more  than  8  to  1 ; 
in  the  common  garden  wistaria  there  are  about  7000  pollen 
grains  to  every  ovule,  and  in  Indian  corn,  the  cone-bearing 
evergreens,  and  a  multitude  of  other  plants,  there  are  many 


FIG.  158.  First  stages  in  the  development  of  the  egg  cell  of  the 
European  ivy  (Hedera  Helix) 

A,  egg  cell.  B :  s,  cell  which  will  form  the  suspensor;  e,  cell  which  will  form  the 
embryo.  C,  showing  first  subdivision  of  the  suspensor-forming  cell ;  D,  show- 
ing subdivision  of  the  embryo-forming  cell;  E,  showing  subdivision  of  both 
regions,  slightly  more  advanced.  —  After  Bonnier  and  Sablon 

times  more  than  7000  to  1.  These  differences  depend  upon  the 
mode  in  which  the  pollen  is  carried  from  the  stamens  to  the 
pistil.  Plants  which  are  pollinated  by  the  wind  must  produce 
far  more  pollen,  to  allow  for  inevitable  waste,  than  those  which 
are  self-pollinated,  or  pollinated  by  insects  (Chapter  xxxn). 


CHAPTEE  XVI 


THE  FRUIT1 

180.  What  constitutes  a  fruit.  It  is  not  easy  to  make  a 
short  and  simple  definition  of  what  botanists  mean  by  the  tejm 
fruit.  It  has  very  little  to  do  with  the  popular  use  of  the 
word.  Briefly  stated,  the  definition  may  be  given  as  follows :  The 
fruit  of  a  seed  plant  consists  of  the  matured  ovary  and  contents, 
together  with  any  intimately  connected  parts.  Botanically  speak- 
ing, the  bur  of  beggar's  ticks  (Fig.  344),  the 
three-cornered  grain  of  buckwheat,  and 
such  true  grains  as  wheat  and  oats  are  as 
much  fruits  as  is  an  apple  or  a  peach. 

181.  Classes  of  fruits.    Fruits  may  be 
divided    into    four     classes    as    follows : 

(a)  unipistillary  fruits,  those  which  re- 
sult from  the  ripening  of  a  single  pistil ; 

(b)  aggregate  fruits,  those   which    result 
FIG.  159.  Group  of  fol-  from  tlie  ripening  of  a  cluster  of  carpels 

licles  and  a  single  of  one  flower,  massed  together;  (c)  acces- 
follicle  of  the  monks-  sory  fruits  those  in  which  the  main  bulk 
hood. 

of  the  fruit  consists  of  something  else  be- 
After  Faguet  .,  ,  ,  ,  . 

sides  the  carpels, —  e.g.  calyx  or  receptacle, 

—  added  to  a  simple  or  an  aggregate  fruit ;  (d)  multiple  or  col- 
lective fruits,  those  which  result  from  the  combination  of  the 
ripened  pistils  of  two  or  more  flowers  into  one  mass. 

182.  Forms  of  unipistillary  fruits :  the  capsule.  This  is  a 
dry  fruit,  splitting  open  (dehiscing]  to  allow  the  seeds  to  escape. 
Capsules  of  simple  pistils  may  either  open  along  one  line,  as 

1  See  Gray,  Structural  Botany,  chap,  vii,  also  Kerner  and  Oliver,  Natural 
History  of  Plant*,  Vol.  II,  pp.  227-438. 

146 


FORMS  OF  UNIPISTILLARY  FRUITS 


147 


in  the  follicles  of  monkshood  (Fig.  159),  or  along  two  lines, 
as  in  the  legumes  of  the  pea  (Fig.  343).  Many  capsules  result 
from  the  ripening  of  compound  pistils,  as  the  poppy,  Datura,  or 
jimson  weed  (Fig.  343),  and  crocus 
(Fig.  166,  I,  B}. 

The  schizocarp.  This  is  a  dry 
fruit,  breaking  into  pieces  which  do 
not  split  open,  the  name  meaning 
breaking  fruit  (Figs.  160,  166,  II). 

The  akene,  grain,  and  nut.  These 
are  dry  fruits  which  never  split 
open  (indehiscent  fruits). 

Under  the  general  name  akene 
are  grouped  several  types  of  fruits. 
Many,  like  those  of  Fig.  161,  are  small  one-seeded  carpels. 
Another  large  group,  the  fruits  of  the  family  Composite,  has 
akenes  which  result  from  the  ripening  of  an  inferior  ovary,  fre- 
quently crowned  by  the  limb  of  the  calyx  (Fig.  166,  III). 


FIG.  160.   Schizocarp  of  maple 
After  Faguet 


r> 


FIG.  161.   Akenes  of  a  buttercup 

A,  head  of  akenes;   B,  section  of 
single  akene  (magnified) ;  a,  seed 


FIG.  162.   Chestnuts 


Grains,  such  as  corn,  wheat,  oats,  barley,  rice,  and  so  on,  have 
the  interior  of  the  ovary  completely  filled  by  the  seed,  and  the 
seed  coats  and  the  wall  of  the  ovary  are  firmly  united,  as  shown 
in  Fig.  3.  Naturally,  therefore,  they  are  popularly  supposed  to 
be  seeds  and  are  always  so  called  by  non-botanical  people. 


148 


THE  FRUIT 


A  nut   (Fig.  162)   is   larger   than   an   akene,  usually  has  a 
harder  shell,  and  commonly  contains  a  seed  which  springs  from 

a  single  ovule  in  one  locule 
of  a  compound  ovary,  which 
develops  at  the  expense  of  all 
the  other  ovules.  The  chestnut 
bur  is  a  kind  of  involucre,  and 
so  is  the  acorn  cup.  The  name 
nut  is  often  incorrectly  ap- 
plied in  popular  language  ;  for 
example,  the  "  Brazil  nut"  is 
really  a  large  seed  with  a  very 


FIG.  163.    Cross  section  of  an  orange 
a,  axis  of  fruit,  with  dots  showing  cut-off 


hard  testa. 

183.  The  berry.    This  is  a 


ends  of  fibro-vascular  bundles ;  p,  par-      generally     fleshy    fruit,    which 
tition  between  cells  of  ovary ;  S,  seed ;  ,,        ,  , . 

c,loculeofovaryfilledwithapulpcom-      usually    does    not    Split    Open, 
posed  of  irregular  sacs  full  of  juice; 
o,  oil  reservoirs  near  outer  surface  of 


Such  berries    as    the    tomato, 


rind ;  e,  corky  layer  of  epidermis.  —      gra<Pe>   and    persimmon    result 
After  Decaisne  from   t]ie   ripening   of    a    supe- 

rior ovary.  Those  of  the  gooseberry,  currant,  and  many  others 
result  from  half-inferior  or  inferior 
ovaries,  and  therefore  a  consider- 
able part  of  the  bulk  of  the  fruit 
is  receptacle.  The  leathery- 
skinned  fruit  of  the  orange  family 
is  a  true  berry. 

The  fruit  of  the  apple,  pear,  and 
quince  is  called  a  pome.    It  con- 
sists of  a  several-loculed  ovary,— 
the  seeds  and  the  tough  membrane 
surrounding  them  in  the  core, — 
inclosed  by  a  fleshy  edible  portion 
which  makes  up  the  main  bulk  of 
the   fruit.     In   the   apple  and  the  pear   much  of  the   fruit  is 
receptacle. 


FIG.  164.  Peach.    Longitudinal 
section  of  drupe 

After  Decaisne 


AGGREGATE  AttD  ACCESSORY  FRUITS  149 

In  the  squash,  pumpkin,  and  cucumber  the  ripened  ovary, 
together  with  the  receptacle,  makes  up  a  peculiar  fruit  (with  a 
firm  outer  rind)  known  as  the  pepp.  The  relative  bulk  of  the 
greatly  enlarged  hollow  receptacle  and  of  the  ovary  in  such 
fruits  is  not  always  the  same. 

The  drupe.  This  fruit  is  often  fleshy,  and  usually  does  not 
split  open.  The  pericarp,  or  wall  of  the  ripened  ovary  (meaning 
round  about  and  fruit),  consists  of  an  outer  fleshy  (or  fibrous 
or  leathery)  layer,  the  exocarp,  and  an  inner,  somewhat  hard  or 
stony  layer,  the  endocarp.  In  common  language  the  endocarp 
with  its  contained  seed  is  called  a  "stone";  hence  drupes  are 


B 

FIG. 165 
A,  strawberry;  B,  raspberry;  C,  mulberry.  —  After  Faguet 

often  known  as  stone  fruits.  Most  drupes,  as  in  the  case  of  the 
peach  (Fig.  164),  cherry,  plum,  cocoanut,  and  walnut,  are  one- 
stoned  and  one-seeded. 

184.  Aggregate  fruits.  The  raspberry  (Fig.  165,1?), blackberry, 
and  similar  fruits  consist  of  many  carpels,  each  of  which  ripens 
into  a  part  of  a  compound  mass  which,  for  a  time  at  least,  clings 
to  the  receptacle.    The  whole  is  called  an  aggregate  fruit. 

185.  Accessory  fruits.   Not  infrequently,  as  in  the  strawberry 
(Fig.  165, A),  the  main  bulk  of  the  so-called  "fruit"  consists 
rather  of  the  receptacle  than  of  the  ripened  ovary  or  its  append- 
ages.   Such  a  combination  is  called  an  accessory  fruit. 

186.  Multiple  fruits.   The  fruits  of  two  or  more  flowers  may 
blend  into  a  single  mass,  known  as  a  multiple  fruit.    Perhaps 


150 


THE  FRUIT 


the  best-known  edible  examples  of  multiple  fruits  are  the  mul- 
berry (Fig.  165,  C)  and  the  pineapple.  The  last-named  fruit  is  an 
excellent  instance  of  the  seedless  condition  which  often  results 
from  long-continued  cultivation. 

Pt 
,-s 


FIG.  166.    Comparative  sections  of  fruits 

I,  capsule :  A,  unilocular  liquorice  pod,  cross  section  (magnified) ;  B,  trilocular 
crocus  pod,  cross  section  (magnified).  II,  schizocarp,  double  fruit  of  poison 
hemlock  (Conium),  cross  section  (magnified) ;  III,  akene  of  arnica,  longitudinal 
section  (magnified) ;  IV,  berry  of  pepper  (Capsicum),  cross  section  (reduced) ; 
V,  drupe  of  cocoanut,  longitudinal  section  (reduced) ;  VI,  aggregate  and  acces- 
sory fruit  of  blackberry,  longitudinal  section  (reduced).  —  I-IV,  after  Schmidt; 
V,  after  Decaisne ;  VI,  (modified)  after  Gray 

c,  limb  of  calyx ;  en,  endocarp ;  ex,  exocarp ;  p,  pericarp ;  pa,  partition  between 
locules ;  r,  receptacle^;  s,  seed 


PAET  II 

THE  MORPHOLOGY,  EVOLUTION,  AND 
CLASSIFICATION  OF  PLANTS 

CHAPTEE  XVII 

THE  PRINCIPLES  OF  MORPHOLOGY,  EVOLUTION,  AND 
CLASSIFICATION 


187.  Morphology.  Morphology  treats  of  the  form  and 
ture  of  a  plant  or  animal.  The  lowest  organisms  have  a  simple 
morphology,  but  the  higher  plants  and  animals  are  made  up 
of  many  parts  or  organs,  and  consequently  their  morphology 
is  very  complex.  Organs  are  structures  set  apart  or  developed 
for  a  definite  kind  of  work.  Thus  the  roots  of  a  plant  are  organs 
usually  employed  to  attach  the  plant  to  the  ground  in  order  that 
it  may  absorb  soil  moisture. 

One  department  of  morphology  (comparative  morphology) 
deals  with  the  various  forms  or  disguises  which  the  same  sort 
of  organ  may  take  in  different  kinds  of  plants,  and  compares 
these  structures  with  one  another.  For  example,  the  foliage  leaf 
is  a  well-defined  organ  which  can  be  recognized  at  a  glance  ;  but 
it  requires  some  study  to  understand  that  the  scales  on  the  bud 
and  around  an  onion,  and  also  some  forms  of  spines  and  tendrils 
are  morphologically  leaves,  that  is  are  leaves  variously  modified. 
Because  all  of  these  structures  are  related  to  one  another  they 
are  called  homologous,  and  morphology  studies  the  homologies,  or 
relationships,  of  organs.  Comparative  morphology  is  one  of  the 
most  interesting  subjects  of  biological  study,  since  it  furnishes 

151 


152     MORPHOLOGY,  EVOLUTION,   AND   CLASSIFICATION 

the  basis  for  the  established  belief  in  the  evolution  or  develop- 
ment of  the  higher  plants  and  animals  from  simpler  forms. 

188.  Classification.  The  classifications  of  animals  and  plants 
are  attempts  to  express  the  actual  kinships,  or  what  among 
human  beings  are  called  blood  relationships,  which  are  believed 
to  exist  among  them.  To  illustrate  the  principles  of  classifica- 
tion let  us  consider  the  position  of  the  pines  among  plants.  All 
of  the  pines  have  for  their  fruit  a  scaly  cone  whose  seeds  are 
borne  naked  at  the  base  of  each  scale  and  mature  the  second 
year.  The  leaves  are  needle-shaped,  evergreen,  and  clustered.  Any 
tree  which  has  all  the  characteristics  above  given  is  a  pine. 

The  spruces,  hemlocks,  firs,  and  larches  agree  with  the  pines 
in  many  respects,  but  all  of  them  mature  their  seeds  the  first 
year,  and  their  foliage  is  different.  The  American  cypress  has  a 
globular  woody  cone  and  deciduous  leaves  in  two  rows.  The 
arbor  vitse  and  the  juniper  have  awl-shaped  or  scale-like  leaves, 
not  in  clusters. 

All  of  these  cone-bearing  trees  are  distinct  kinds,  but  they 
are  grouped  together  because  the  seeds  are  borne  naked  on  the 
scales  of  the  cones.  This  peculiarity  separates  the  group  from 
a  much  larger  assemblage  of  seed  plants  in  which  the  seeds 
are  borne  inclosed  in  seed  cases,  pods,  or  other  types  of  fruit. 
Finally,  all  of  the  s<?ec?-bearing  plants  are  separated  from  the 
spore-bearing  groups  by  the  possession  of  methods  of  repro- 
duction which  develop  seeds. 

Thus  the  pines  find  their  place  in  the  classification  of  plants 
through  clearly  marked  characters  which  define  several  different 
groups.  These  characters  are  (1)  the  presence  of  the  seed,  (2)  the 
fact  that  the  seeds  are  exposed  or  naked,  (3)  the  development  of 
the  seeds  in  a  cone  type  of  fruit,  and  finally  (4)  some  peculiar- 
ities of  the  cone,  and  the  character  of  the  foliage.  The  process 
of  classification  leads  from  an  assemblage  of  more  than  one 
hundred  thousand  kinds  of  plants  (the  seed  plants),  through 
successively  smaller  divisions,  to  the  relatively  small  group  of 
the  pines,  with  hardly  more  than  seventy  known  kinds. 


ORGANIC  EVOLUTION  153 

189.  Nomenclature.    It  was  long  ago    found    convenient  to 
give  Latin  names  to  the  kinds  of  animals  and  plants  and  to  their 
various  natural  groups.    These  names  constitute  the  nomencla- 
ture of  botany  and  zoology.    Each  kind  of  plant  or  animal  is 
termed  a  species.    A  group  of  closely  related  species  constitutes 
a  genus  (plural,  genera).    Every  species  is  given  a  name  that 
consists  of  two  parts.    There  is  the  specific  name,  which  defines 
the  species,  and  the  generic  name,  which  includes  the  more  im- 
mediate relatives.    The  specific  name  follows  the  generic,  just  as 
the  first  name  of  a  man  follows  his  family  name  or  his  surname 
in  a  directory.    Furthermore,  an  abbreviation  of  the  name  of  the 
botanist  who  first  described  the  species  follows  the  combination 
of  generic  and  specific  names.    Thus  the  name  of  the  pitch  pine 
is  written  Pinus  rigida  Mill.,  this  species  having  been  described 
by  a  botanist  named  Miller.    This  universally  adopted  system  of 
designating  species  by  two  names,  known  as  the  binomial  system 
of  nomenclature,  was  perfected  by  the  famous  Swedish  natu- 
ralist Linnaaus,  and  the  edition  of  his  Species  Plantarum,  which 
is  the  basis  of  all  botanical  classification,  bears  the  date  1753. 

Closely  related  genera  are  grouped  into  families,  whose  names 
generally  terminate  in  the  ending  -acece,  and  families  are  brought 
together  into  orders,  whose  names  are  written  with  the  uniform 
ending  -ales.  Orders  are  further  assembled  into  classes,  and  the 
classes  into  subdivisions,  or  more  frequently  into  divisions,  of 
the  plant  kingdom.  Applying  this  system  of  classification,  we 
ha.ve  all  the  species  of  pines  in  the  genus  Pinus,  in  the  family 
Pinacece,  in  the  order  Conifer  ales,  in  the  class  Coniferce,  in  the 
subdivision  Gymnospermw  of  that  highest  division  of  the  plant 
kingdom,  the  Spermatophyta. 

190.  Organic  evolution.    In  the  times  of  Linnaaus,  who  lived 
in  the  eighteenth  century,  almost  all  naturalists  believed  that 
the  species  or  kinds  of  animals  and  plants  had  never  changed 
in  their  characters  during  their  long  history  on  the  earth.    They 
believed  that  new  kinds  could  only  arise  by  special  acts  of  cre- 
ation.   This  doctrine  of  special  creation  gave  way  to  the  present 


154     MORPHOLOGY,   EVOLUTION,  AND  CLASSIFICATION 

belief  in  organic  evolution,  or  the  theory  of  descent,  chiefly  through 
the  work  of  Charles  Darwin,  whose  famous  book,  The  Origin  of 
Species,  appeared  in  1859.  The  theories  of  organic  evolution 
hold  that  all  the  existing  species  of  animals  and  plants  have 
been  derived  or  evolved  through  the  geological  ages  from  the 
simplest  forms  of  life  in  the  beginning.  These  theories  also 
hold  that  the  kinds  now  on  the  earth  are  subject  to  change,  and 
that  very  many  of  them  are  in  process  of  developing  new  species. 
There  are  varying  opinions  as  to  the  causes  which  bring  about 
changes  in  species,  and  there  are  several  schools  of  evolutionists 
whose  theories  are  the  subject  of  constant  discussion  arid  inves- 
tigation.1 But  all  botanists  and  zoologists  believe  in  the  main 
principles  of  organic  evolution  ;  and  the  theory  is  the  framework 
of  biology.  Indeed,  the  theory  of  organic  evolution  is  as  impor- 
tant to  biology  as  the  atomic  theory  is  to  chemistry  and  as  the 
doctrine  of  the  conservation  of  energy  is  to  physics. 

191.  An  outline  of  the  classification  of  plants.  We  shall 
present  at  this  point  a  classified  arrangement  *of  the  most  im- 
portant of  the  larger  groups  of  plants.  It  is  quite  impossible 
to  develop  a  classification  very  far  in  the  compass  of  this  book, 
but  this  outline  will  serve  to  indicate  the  field  covered  in  the 
succeeding  chapters.2  The  thallophytes  are  especially  difficult 
to  classify,  for  the  groups  are  not  as  clearly  understood  as  those 
of  the  higher  plants,  and  there  are  complex  relationships,  espe- 
cially between  the  algse  and  the  fungi.  The  classification  of 
the  green  algse  offers  exceptionally  difficult  problems,  and  the 
arrangement  presented  here  is  largely  one  of  convenience  in  the 
present  state  of  our  knowledge  of  this  puzzling  assemblage  of 
forms.  Classifications  are,  of  course,  subject  to  constant  modi- 
fication, as  groups  receive  more  and  more  careful  study,  and 
authors  frequently  differ  widely  in  their  systems. 

1  See  Chapter  xxxix,  Variation,  Mutation,  and  Origin  of  Species. 

2  For  the  most  recent  and  detailed  classification  of  plants  the  reader  is 
referred  to  Engler,  Syllabus  der  Pflanzenfamilien,  1903,  or  to  Engler  and 
Prantl,  Die  Naturlichen  Pflanzenfamilien. 


CLASSIFICATION  OF   PLANTS  155 

AN  OUTLINE  OF  THE  CLASSIFICATION  OF  PLANTS 
DIVISION  I.    Thallophyta,  the  thallus  plants,  or  thallophytes. 

SERIES  or  THE  ALG^E. 

CLASS     I.    Cyanophycece,  the  blue-green  algae.  , 
II.    Chlorophycece,  the  green  algae. 

Order  1.    Protococcales,  the  one-celled  green  algae. 

2.  Confervales,  the  confervas  and  sea  lettuce. 

3.  Conjugates,  the  pond  scums 

4.  Diatomales,  the  diatoms. 

5.  Siphonales,  the  siphon  algae. 

6.  Charales,  the  stoneworts. 

III.  Phceophycece,  the  brown  algae. 

IV.  Rhodophycece,  the  red  algae. 

SERIES  OF  THE  FUNGI. 

CLASS    V.    Schizomycetes,  the  bacteria. 
VI.    Saccharomycetes,  the  yeasts. 
VII.    Phycomycetes,  the  alga-like  fungi. 
VIII.    Ascomycetes,  the  sac  fungi. 
IX.    Basidiomycetes,  the  basidia  fungi. 

D  [VISION  II.    Bryophyta,  the  liverworts  and  mosses,  or  bryophytes. 
CLASS     I.    Hepaticce,  the  liverworts. 

Order  1.    Ricciales,  the  Riccia  forms. 

2.  Marchantiales,  the  Marchantia  forms. 

3.  Jungermanniales,  the  Jungermannia  forms,  or 

leafy  liverworts. 

4.  Anthocerotales,  the  Anthoceros  forms. 
II.    Musci,  the  mosses. 

Order  1.    Sphagnales,  the  peat  mosses. 
2.    Bryales,  the  common  mosses. 

DIVISION  III.    Pteridophyta,  the  ferns  and  their  allies,  or  pteridophytes. 
CLASS     I.    Filicinece,  the  true  ferns. 
II.    Equisetinece,  the  horsetails. 
III.    Lycopodinece,  the  club  mosses. 

DIVISION  IV.    Spermatophyta,  the  seed  plants,  or  spermatophytes. 
SUBDIVISION     I.    Gymnospermce,  the  gymnosperms. 

II.    Angiospermce,  the  angiosperms.1 
CLASS    I.    Monocotyledoneoe,,  the  monocotyledons. 
II.    Dicotyledonece,  the  dicotyledons. 

1  The  reader  should  note  that  in  this  classification  the  angiosperms  con- 
tain only  two  out  of  sixteen  classes  of  somewhat  equivalent  value. 


CHAPTER  XVIII 

THE  LOWEST  ORGANISMS  AND  THE  CELL  AS  THE 
LIFE  UNIT 

192.  The  process  of  evolution.    The  higher  complex  animals 
and  plants  are  readily  distinguished  from  one  another,  but  the 
differences  become  less  apparent  in  the  lower,  simpler  forms. 
There  are  indeed  groups   of   uncertain  position,  some  authors 
placing  them  among  the  plants  and  some  among  the  animals. 

The  animal  and  plant  kingdoms,  in  the  process  of  evolution, 
followed  a  tree-like  method  of  development.  The  forms  and 
groups  split  up  into  divergent  lines  which  constantly  gave  off, 
and  are  still  giving  off,  new  shoots.  Thus  from  a  number  of 
trunks  in  the  beginning  there  have  been  derived  a  multitude 
of  smaller  branches,  and  from  these  in  turn  have  arisen  count- 
less twigs.  It  is  impossible  to  construct  accurately  these  genea- 
logical trees,  because  the  species  now  living  occupy  the  position 
of  buds  on  the  structure,  some  relatively  low  down  and  some  at 
the  highest  points,  but  all  at  the  ends  of  their  respective  lines 
of  development.  The  forms  which  represented  the  lowest  and 
intermediate  stages  of  development  are  almost  all  extinct, —  that 
is,  have  long  ago  died  out  on  the  earth,  —  and  we  can  only  judge 
of  their  structure  by  the  fragmentary  remains  which  are  left  as 
fossils,  or  by  comparative  studies  on  the  structure  and  develop- 
ment of  living  species,  which  frequently  give  us  suggestions  of 
what  took  place  in  the  long  process  of  organic  evolution. 

193.  The  simplest  living  unit  a  cell.    The  living  material  of 
organisms,  that  is  the  part  which  possesses  life,  is  called  pro- 
toplasm.    Protoplasm  is  not  a  simple  substance,  but,  on  the 
contrary,  is  the  most  involved  mixture  of  the  most  complex 
substances  which  the  chemist  knows.    These  belong  to  the  group 

166 


THE   SIMPLEST  LIVING  UNIT  A  CELL  157 

called  proteids,  a  familiar  example  of  which  is  the  white  of  egg 
(albumen).  Very  little  is  known  of  the  exact  chemical  structure 
of  the  numerous  proteids.  Their  molecules  are  very  complex, 
for  they  contain  a  large  number  of  elements  of  remarkably 
varied  chemical  properties,  —  carbon,  nitrogen,  oxygen,  hydro- 
gen, sulphur,  and  in  some  cases  phosphorus.  But  besides  the 
proteids  and  many  other  organic  compounds  (substances  usually 
formed  only  in  association  with  life  processes,  as,  for  example, 
the  sugars,  starch,  and  fats),  protoplasm  also  contains  certain 
necessary  inorganic  substances,  such  as  salts  of  sodium,  potas- 
sium, calcium,  magnesium,  and  iron,  and  in  addition  to  these  a 
very  large  amount  of  water. 

Although  we  know  very  little  about  the  chemical  nature 
of  protoplasm,  certain  remarkable  structural  peculiarities  have 
been  recognized  for  a  long  time.  Protoplasm  always  exists  in 
the  form  of  units  which  are  called  cells.  The  simplest  organisms 
consist  of  solitary  units,  and  are  consequently  termed  one-celled 
(unicellular).  The  higher  organisms  are  made  up  of  aggregates 
of  cells,  and  are  termed  many-celled  (multicellular). 

The  cells  in  many-celled  organisms  have  each  a  separate  indi- 
viduality, but  they  are  usually  set  apart  for  particular  kinds  of 
work  and  depend  upon  one  another  for  mutual  assistance.  The 
many-celled  organism  has  been  termed  a  cell  republic,  because 
all  the  cells,  as  individuals,  work  for  the  common  good  of  the 
community,  and  by  a  system  of  helpful  division  of  labor  benefit 
one  another. 

There  is  a  large  group  of  one-celled  microscopic  animals  called 
the  Protozoa.  This  constitutes  the  lowest  division  of  the  animal 
kingdom,  and  is  quite  distinct  from  the  groups  of  many-celled 
animals,  although  they  are  believed  to  have  been  derived 
from  it.  There  are  likewise  numerous  one-celled  plants,  but 
they  are  related  to  the  higher  many-celled  forms  by  very  com- 
plete and  interesting  connecting  links,  so  that  botanists  do  not 
make  a  group  of  one-celled  plants,  and  can  readily  understand 
the  evolution  of  the  many-celled  forms  from  the  single-celled. 


158 


THE  LOWEST  ORGANISMS  AND  THE  CELL 


It  is  much  easier  to  understand  the  structure  of  the  plant 
cell  by  comparing  it  with  one  of  the  simplest  of  the  one-celled 
animals  ;  so  at  this  point  there  will  be  given  a  brief  account  of 
one  of  the  best-known  protozoans,  the  Amoeba. 

194.  The  Amoeba.*  The  Amoeba  under  the  microscope  appears 
as  a  minute,  irregularly  shaped  body  of  a  jelly-like  consistency. 
Its  form  when  active  constantly  changes.  A  finger-like  extension 

or  process  is  thrust 
out  from  one  side 
(Fig.  167,^)  and  the 
somewhat  granular 
protoplasm  flows  into 
this  from  neighbor- 
ing regions.  Other 
processes  are  succes- 
sively withdrawn,  so 
that  the  protoplasm 
actually  moves  or 
flows  slowly  forward 
into  the  newly  formed 
lobe,  and  thus  the 
Amceba  glides  along. 
There  is  present  in  the 
protoplasm  a  denser 
protoplasmic  struc- 
ture termed  the  nucleus,  which  is  known  to  be  the  center  of 
very  important  activities  in  the  cell.  The  protoplasm  also  con- 
tains numerous  small  granules,  and  frequently  large  food  parti- 
cles, and  there  are  also  globules,  called  vacuoles,  of  a  watery 
fluid,  which  appear  and  disappear  in  the  thicker  substance. 
Such  is  the  structure  of  a  typical  cell,  which  may  be  defined  as 
a  small  mass  of  protoplasm  containing  a  nucleus. 

*  To  THE  INSTRUCTOR  :  If  material  of  Amceba  is  available,  its  study  will 
furnish  an  excellent  introduction  or  accompaniment  to  laboratory  work  on 
the  plant  cell. 


FIG.  167.  The  Amceba 

A,  an  individual  moving  in  the  direction  of  the 
arrows;  n,  nucleus  ;  v,  pulsating  vacuole;  /,  food 
body.  B,  the  process  of  cell  division  by  constric- 
tion, a  nucleus  in  each  half. — H,  after  Jordan, 
Kellogg,  and  Heath 


THE  PLANT  CELL  159 

The  Amoeba  feeds  upon  smaller  organisms.  These  may  be 
drawn  in  at  any  point  on  the  surface  of  the  cell,  whose  proto- 
plasm simply  flows  around  the  bodies  and  thus  takes  them  into 
the  interior.  The  oxygen  gas  held  in  the  water  which  bathes 
the  Amoeba  is  also  absorbed  all  over  its  surface.  Food  materials 
which  cannot  be  digested,  together  with  the  waste  products,  are 
left  behind  by  the  protoplasm  as  it  moves  from  place  to  place. 

When  the  Amoeba  reaches  a  certain  size  there  takes  place 
the  interesting  event  called  cell  division.  The  cell  divides, 
by  a  process  of  constriction  (Fig.  167,  B},  into  similar  halves, 
which  separate  from  one  another  as  two  independent  daughter 
Amoeboe.  Previous  to  the  division  of  the  cell  there  has  been  a 
division  of  the  nucleus,  so  that  each  daughter  Amoeba  is  pro- 
vided with  a  daughter  nucleus,  and  therefore  has  exactly  the 
same  structure  as  the  parent  cell,  but  is,  of  course,  only  about 
half  as  large.  Cell  division  is  the  method  of  cell  reproduction. 
It  is  interesting  to  note  that  in  this  process  of  reproduction 
there  has  been  no  loss  of  protoplasm,  no  death  of  any  region  of 
the  parent  Amoeba,  but  from  the  division  of  one  have  come  two. 
There  is,  therefore,  no  death  from  old  age  in  one-celled  organ- 
isms. They  are  being  killed  constantly,  of  course,  by  adverse 
conditions,  or  eaten  by  other  animals.  These  are  the  accidents 
of  life.  However,  the  Amoeba  and  other  one-celled  animals  and 
plants  need  never  die  of  old  age ;  that  is,  there  is  nothing  in  the 
constitution  of  such  an  organism  to  prevent  its  living  forever. 

195.  The  plant  cell.  The  plant  cell  generally  differs  from 
the  animal  cell  in  two  important  respects. 

First.  The  protoplasm  is  inclosed  in  a  little  box-like  chamber 
with  transparent  walls.  The  substance  of  the  walls  is  called  cellu- 
lose, —  a  compound  belonging  to  the  great  group  of  the  starches 
and  sugars  (carbohydrates).  Such  an  envelope  is  termed  a  cell 
wall,  and  is  peculiar  to  plants.  Indeed,  the  term  cells,  as  used  in 
biology,  was  first  applied  to  the  chambers  inclosed  by  cell  walls, 
which  may  be  seen  in  thin  sections  of  cork,  pith,  and  other 
plant  structures. 


160          THE  LOWEST  ORGANISMS  AND  THE  CELL 


Second.    The  protoplasm  of  green  portions  of  plants  will  be 

found  to  contain  green  bodies  called  chromatopliores  (meaning 

color  bearers).    Chromatopliores  have  a  great  variety  of  forms  in 

different  plants  and  are  sometimes  very  complex  and  beautiful, 

—  as  the  spiral  band  in  the  cells  of  the  pond  scum,  Spirogyra 

(Fig.  168,  A).  The  green  color- 
ing matter  in  a  chromatophore 
is  called  chlorophyll  (meaning 
leaf  green).  Green  chromato- 
pliores are  called  chloroplasts 
when  small  and  numerous  in 
a  cell.  Chloroplasts  are  char- 
acteristic of  the  cells  in  plants 
above  the  thallophytes,  and 
may  be  readily,  studied  in  the 
leaves  of  mosses  (Fig.  169,  A), 


ferns,  and  seed  plants.  Chro- 
matopliores are  sometimes  col- 
ored brown  or  red,  as  in  the 
cells  of  the  brown  and  the  red 


algae.    Chromatophores  are 


FIG.  168.   Cell  structure  of  the  pond 
scum  (Spirogyra) 

A,  living  cell,  showing  spiral  band-like    peculiar  to  plants,  never  being 

chromatophore  with  pyrenoids  p,  and  found  in  typical  animal  cells. 

centrally  placed  nucleus  n;  fi,  living 

cell  after  treatment  with  a  salt  solu-  The  protoplasm  of  the  plant 

tion,  the  protoplasm  contracted  away  U  alw          H       directly  Under 

from  the  cell  wall ;  C,  pyrenoid  stained  J                             J 

with  iodine  and  very  greatly  magni-  the    Cell  wall,  Sometimes   COm- 

fied  (ahout  1000  diameters),  a  circle  of  ^Ipfplv    fillino-   thp    ravirv    hnf- 

starch  grains  around  the  pyrenoid  ?i€                                      cavity,    but 

more  frequently  forming  a  lin- 
ing which  surrounds  one  or  more  spaces,  or  vacuoles,  which 
contain  a  watery  fluid  called  cell  sap.  The  relation  of  the  proto- 
plasm to  the  cell  wall  is  easily  understood  when  the  protoplasm 
is  made  to  contract  from  the  wall  by  the  withdrawal  of  the 
watery  cell  sap  from  the  vacuoles.  Thus  if  a  filament  of  a  pond 
scum  or  a  portion  of  a  moss  leaf  be  placed  in  an  aqueous  solu- 
tion of  common  salt  (5  or  10  per  cent),  the  cell  sap  is  drawn 


THE  PLANT  CELL 


161 


J 


out  of  the  vacuole  (osmotically)  and  the  bounding  layer  of  proto- 
plasm shrinks  away  from  the  wall  (Figs.  168,  7?;  169,1?).  The 
force  that  keeps  the  layer  of  protoplasm  against  the  cell  wall  is 
called  cell  turgor. 

The  mass  of  protoplasm  inclosed  by  the  cell  wall  is  called 
the  protoplast,  and  always  contains  at  least  one  nucleus.  Some 
plant  cells  have  many 
nuclei.  The  posi- 
tion of  the  nucleus 
is  somewhat  variable. 
In  the  cell  of  the  pond 
scum  (Fig.  168,  A,  a) 
it  is  situated  in  the 
middle  region  and 
held  in  place  by  deli- 
cate strands  of  proto- 
plasm which  run  out 
to  the  protoplasmic 
layer  under  the  cell 
wall,  but  the  nucleus 
frequently  lies  just 

under  the  wall,  as  in          FIG.  169.  Cell  structure  of  the  moss  leaf 

(Funaria) 


,  two  living  cells  from  a  leaf,  showing  the  numer- 
ous chloroplasts  and  the  position  of  the  nucleus 
n  in  the  layer  of  protoplasm  under  the  cell  wall ; 

B,  living  cell  after  treatment  with  a  salt  solution, 
the  protoplast  contracted  away  from  the  cell  wall ; 

C,  stages  illustrating  the  division  of  the  plastids, 
starch  grains  shown  in  their  interiors 


the  moss   leaf    (Fig. 

matophores  are  gen- 
erally found  in  the 
outer  layer  of  proto- 
plasm under  the  cell 
wall.  There  are  also  many  granules  in  the  protoplasm,  some  of 
them  minute  globules  of  oils  and  fats  and  others  of  a  proteid 
character.  Many  of  these  are  food  products  in  the  cell.  Finally, 
the  central  portion  of  the  cell  generally  contains  a  single  vacuole 
filled  with  cell  sap. 

It  is  clear  that  the  protoplast  of  the  plant  cell  corresponds  to 
the  entire  Amozbq,  or  any  other  animal  cell.    The  cell  wall  is  a 


162         THE  LOWEST   ORGANISMS  AND  THE   CELL 

formation  outside  of  the  protoplast  and  is  not  a  living  part 
of  the  plant  cell.  Many  lower  plants  form  reproductive  cells 
(zoospores,  gametes,  etc.),  which  for  some  time  are  without  a 
cellulose  wall,  and  in  this  condition  are  motile  and  behave  like 
animal  cells.  However,  the  cell  walls  and  the  chromatophores 
are  responsible  for  the  most  conspicuous  differences  between 
plants  and  animals,  as  is  noted  in  Sec.  202. 

196.  Photosynthesis.1  Chromatophores  and  chloroplasts  in 
the  presence  of  sunlight  are  able  to  manufacture  from  water 
and  the  simple  gas  carbon  dioxide  certain  complex  organic 
foods  of  which  starch  is  generally  the  first  visible  product. 
This  process  is  called  photosynthesis,  which  signifies  a  putting 
together  by  light.  The  chemical  formula  for  carbon  dioxide  is 
C02,  for  water  H2O,  and  for  starch  C6H1005.  The  chemistry  of 
the  manufacture  of  starch  cannot  be  truthfully  shown  by  a 
simple  equation,  for  starch  is  not  formed  directly  from  carbon 
dioxide  and  water,  but  by  several  steps  through  invisible  sub- 
stances that  have  not  been  isolated  and  therefore  have  never 
been  studied.  The  chemical  processes  in  these  steps  are  not 
well  understood.  The  final  results  may  be  roughly  expressed 
as  follows : 

6  C02  +  5  H20  =  C6H1005  +  6  02. 

This  shows  why  free  oxygen  is  formed  during  the  processes  of 
photosynthesis.  In  some  plants  starch  is  never  manufactured, 
but  instead  sugars,  which  are  substances  closely  related  to 
starch,  some  of  them  having  the  formula  C6H1206.  The  sugars 
are  in  solution  and  invisible.  Oil  is  formed  in  some  plants,  as 
in  the 'green  felt  (Vaucheria),  diatoms,  etc.,  in  place  of  starch,  as 
the  first  visible  product  of  photosynthesis. 

Many  chromatophores  have  well-defined  denser  regions  called 
pyrenoids,  which  are  the  centers  of  starch  formation,  as  is 
well  illustrated  in  the  pond  scum  (Fig.  168,  C).  Chloroplasts 

1  The  subject  of  photosynthesis  is  treated  in  greater  detail  in  connection 
with  the  structure  and  functions  of  leaves  (Chapter  xn),  especially  in 
Sees.  127-132. 


THE  FOOD  OF  PLANTS;   ASSIMILATION  163 

frequently  contain  starch  grains,  as  may  be  readily  shown  in 
the  cells  of  the  moss  leaf  (Fig.  169,  C)  when  colored  (stained) 
with  iodine.  Photosynthesis  is  only  found  in  plants  containing 
chlorophyll  or  other  pigments  of  a  similar  physiological  nature. 
The  sun  furnishes  the  energy  in  the  form  of  light  for  the  build- 
ing up  of  the  simplest  food  products,  and  the  plant  cell  is  the 
main  factory  which  supplies  the  food  of  the  world. 

197.  The   food   of   plants ;    assimilation.    All  plants  with 
chlorophyll  can  manufacture  their  own  food  by  the  processes  of 
photosynthesis.    Moreover,  it  is  manufactured  directly  within 
the  protoplasm  of  the  cell  and  does  not  have  to  be  absorbed 
from  without,  as  in  the  case  of  the  animal  cell  (see  account 
of  Amoeba,  Sec.  194). 

As  we  have  already  noted,  starch  is  generally  the  first  visible 
product  of  this  process  of  food  manufacture  (photosynthesis). 
Starch  and  the  related  substances,  sugars,  are  the  primary  foods 
of  green  plants,  and  the  most  important,  but  they  are  merely  the 
starting  point  for  a  complex  series  of  processes  through  which 
the  highly  organized  proteids  of  the  protoplasm  are  derived. 
There  are  some  plants  which  lack  chlorophyll,  as  the  fungi  and 
certain  plant  parasites,  and  they,  like  the  animals,  depend  upon 
food  absorbed  from  without  the  body.  The  food  of  plants  is 
broken  down  and  recombined  in  various  ways  to  form  the  pro- 
toplasm, as  it  is  in  animals,  and  the  breaking  down  of  some  of 
the  substances  sets  free  energy  in  the  form  of  plant  heat  (corre- 
sponding to  animal  heat),  as  is  easily  proved  in  the  germination 
of  seeds  (see  Sec.  5).  So  the  processes  of  food  absorption,  or 
assimilation,  in  plants  are  essentially  the  same  as  in  animals, 
but  the  manufacture  of  food  (photosynthesis)  is  an  entirely  dif- 
ferent process  and  peculiar  to  plants. 

198.  The  food  cycle.    There  is  a  circulation  of  certain  ele- 
ments (especially   carbon,  nitrogen,  sulphur,  and  phosphorus) 
through  the  bodies  of  plants  and  animals  which  may  be  called 
the  food  cycle  (see  diagram,  Fig.  207).    It  begins  in  the  plant 
cell  with  the  manufacture  of  starch,  and  related  substances 


164         THE  LOWEST  ORGANISMS  AND  THE  CELL 

(carbohydrates)  by  photosynthesis.  This  makes  carbon,  obtained 
from  the  carbon  dioxide  of  the  air,  available  in  these  primary 
foods.  Nitrogen  is  obtained  from  the  nitrates  dissolved  in  water 
and  drawn  up  through  the  roots,  and  sulphur  and  phosphorus  in 
a  similar  manner  from  sulphates  and  phosphates.  The  proteids 
of  protoplasm  are  built  up  from  these  elements,  with  the  addi- 
tion of  hydrogen  and  oxygen.  Plants  are  able  to  form  some 
very  complex  organic  substances,  but  animals  are  able  to  carry 
the  building-up  process  still  farther,  for  the  highest  forms  of 
proteids  known  are  found  in  their  substance. 

There  is,  however,  a  turning  point  in  the  building-up  activi- 
ties when  complex  compounds  begin  to  break  down  into  simpler 
substances.  Some  of  these  are  the  daily  waste  products  of 
an  animal  or  plant.  The  most  striking  phenomena  are  those 
which  occur  during  the  processes  of  decay,  which  begin  at  once 
with  the  death  of  an  organism.  Decay  is  the  process  by  which 
highly  complex  organic  compounds  are  broken  down  into  suc- 
cessively simpler  substances.  The  final  steps  return  the  ele- 
ments carbon,  nitrogen,  sulphur,  and  phosphorus  to  the  earth 
and  air  in  very  simple  forms  available  again  for  the  constructive 
work  of  green  plants.  The  processes  of  decay  are  due  to  the 
growth  and  activities  of  bacteria  and  other  fungi,  and  the  sub- 
ject is  treated  at  some  length  in  Sec.  252. 

199.  Cell  division,  growth,  and  reproduction.  Assimilation 
increases  the  amount  of  protoplasm,  and  this  results  in  growth 
and  reproduction  through  cell  and  nuclear  division.  Cell  divi- 
sion in  plants,  as  in  animals,  is  preceded  by  nuclear  division, 
after  which  a  cell  wall  is  formed  between  the  daughter  pro- 
toplasts. The  nucleus  in  the  resting  condition  contains  granular 
material  called  chromatin,  which  may  be  readily  colored  (stained) 
by  certain  dyes.  Generally  there  are  also  present  one  or  more 
globular  bodies  called  nucleoles  (Fig.  170,  A).  Chromatin  is  a 
proteid  and  is  believed  to  be  the  essential  substance  of  the 
nucleus  and  necessary  for  the  life  of  the  cell,  because  protoplasm 
will  not  live  if  deprived  of  nuclei.  Just  previous  to  nuclear 


THE  CELL  THEORY   OF  ORGANIZATION 


165 


division  the  chromatin  becomes  organized  into  a  number  of 
bodies  called  chromosomes,  each  of  which  splits,  and  the  halves 
are  distributed  in  two  sets  to  the  daughter  nuclei.  The  distri- 
bution of  the  chromosomes  is  effected  by  an  interesting  appa- 
ratus called  a  spindle  (Fig.  170,  B),  which  consists  of  delicate 
fibers  (spindle  fibers)  formed  in  the  early  stages  of  nuclear 
division.  The  two  sets  of  daughter  chromosomes  (Fig.  170,  C) 
collect  at  the  poles  of  the  spindle  to  organize  the  daughter 
nuclei,  which  then  pass  into  the  resting  condition  (Fig.  170,  D), 
and  a  cell  wall  is  formed  between,  that  divides  the  original 


FIG.  170.   Stages  in  nuclear  and  cell  division  from  the  root  tip  of  an  onion 

A,  resting  nucleus  with  the  chromatin  in  the  form  of  a  network  and  two  nucleoles ; 
B,  a  spindle  with  the  divided  chromosomes  gathered  in  the  middle  region  and 
about  to  separate  into  two  groups  of  daughter  chromosomes ;  (7,  the  two  sets  of 
daughter  chromosomes  at  the  poles  of  the  spindle ;  D,  formation  of  the  new  wall 
between  the  daughter  nuclei 

cell  into  two  daughter  cells.  It  is  a  remarkable  fact  that  the 
number  of  chromosomes  in  the  nucleus  is  fixed  for  different 
plants,  —  a  point  which  we  shall  have  occasion  to  consider  in 
other  connections. 

Chromatophores  reproduce  themselves  by  simple  fission,  or 
splitting,  very  plainly  illustrated  in  the  cells  of  the  moss  leaf 
(Fig.  169,  C),  and  are  thus  passed  on  with  each  cell  division. 

200.  The  cell  theory  of  organization.  The  process  of  growth 
and  development  of  a  many-celled  organism  is  through  con- 
tinuous cell  multiplication.  Development  generally  begins  with 
a  cell,  which  both  in  animals  and  in  plants  is  called  the  egg. 


166 


THE  LOWEST  ORGANISMS  AND  THE  CELL 


The  egg  is  a  female  sexual  element  which  normally  cannot 
develop  into  a  new  organism  until  a  male  sexual  cell,  called 
the  sperm,  has  united  with  it.  This  union  is  called  fertilization  ^ 
and  the  fertilized  egg  is  a  sexually  formed  cell  because  it  results 

from  the  fusion  of  two  sexual  cells, 
the  egg  and  sperm.  The  fertilized 
egg  is  termed  an  oospore  (meaning 
an  egg  spore),  when  there  is  a  rest- 
ing period  before  its  further  develop- 
ment, or  germination.  A  sexual  cell, 
such  as  the  egg  or  sperm,  is  called 
a  gamete.  The  protoplasmic  union 
of  egg  and  sperm  is  very  complete, 
for  the  two  nuclei  come  together  in 
the  center  of  the  egg  and  fuse  to 
form  a  large  nucleus  which  has,  of 
course,  about  twice  as  much  of  that 
important  nuclear  substance,  chro- 
matin,  as  the  single  nucleus  of  either 
egg  or  sperm. 

Frequently  there  are  present  in 
plants  other  forms  of  reproductive 
cells  called  spores,  which  are  not 
formed  sexually  but  are  simply  spe- 
cial  cells  which  can  develop  at  once 

zygospores ;  B,  another  species     into  new  plants. 

(S.  lonqata),  in  which  the  cell  rrn  •  p 

unions  occur  between  adjacent  The    umon     of    gametes    to    give 

gametes  in  the  same  filament.—    sexually  formed  cells  is  especially 

After  Schenck  ,,    .,,  ,     ,    .       ,,        ,,      ...        i      <? 

well  illustrated  in  the  fruiting1  of 

the  pond  scum  (Spirogyra).  In  most  species  the  cells  of  fila- 
ments lying  side  by  side  put  forth  short  processes  which  fuse  in 
pairs,  thus  presenting  a  characteristic  ladder-like  arrangement 
(Fig.  171,  A).  The  contents  of  one  cell  then  pass  over  and 

1  The  terms  fruit  and  fructification  will  be  used  in  Part  II  in  an  untech- 
nical  sense  to  designate  various  forms  of  reproductive  organs  and  processes. 


FIG.  171.   The  union  of  the 
gametes  in  Spirogyra 

A,  two  filaments  of  Spirogyra 
quinina,  side  by  side,  show- 


PROPERTIES  PECULIAR  TO  LIVING  MATTER       167 

unite  with  that  of  the  other,  giving  a  large  fusion  protoplast 
which  develops  a  heavy  protective  wall  and  is  a  sexually  formed 
spore.  It  is  called  a  zygospore  (meaning  yoked  spore)  because 
the  gametes  are  similar,  like  the  halves  of  a  yoke.  This  cell 
union  is  the  same  in  all  essentials,  including  the  final  fusion  of 
the  two  nuclei,  as  the  fertilization  of  the  egg,  except  that  the 
two  sexual  cells,  or  gametes,  are  not  different  in  form  as  are 
eggs  and  sperms  (examine  illustrations  of  Volvox  (Fig.  178), 
(Edogonium  (Fig.  182),  Fucus  (Fig.  199),  etc.).  The  fruiting  of 
Spirogyra  is  a  relatively  simple  illustration  of  a  sexual  process, 
for  the  gametes  are  similar  and  have  never  become  differen- 
tiated into  eggs  and  sperms. 

Development  proceeds  through  continued  cell  divisions,  which 
lead  to  growth  and  a  gradual  specialization  or  setting  apart  of 
certain  cells  for  particular  kinds  of  work  in  the  body.  This 
specialization  of  cells  results  in  the  various  forms  of  cell  struc- 
tures, or  tissues,  of  the  mature  organism.  So  the  life  history  is  a 
succession  of  cell  divisions,  and  the  reproduction  of  the  species 
is  a  return  to  a  one-celled  condition  through  the  reproductive 
cells  (gametes  and  spores).  The  animal  and  plant  body  dies, 
but  the  stream  of  life  flows  on  through  the  reproductive  cells. 
This  is  the  outline  of  the  cell  theory  of  organization,  which 
perhaps  ranks  next  to  the  theory  of  organic  evolution  as  one  of 
the  fundamental  principles  of  biological  science. 

201.  Properties  peculiar  to  living  matter.  We  have  noted 
that  the  chemical  composition  and  reactions  of  protoplasm  are 
exceedingly  complex,  but  nevertheless  there  are  no  reasons  for 
supposing  that  they  are  outside  of  chemical  and  physical  laws. 
However,  protoplasm  has  properties  which  distinguish  it  from 
lifeless  matter  (see  also  Sees.  45-47). 

Protoplasm  has  the  power  of  growth  and  repair.  This  means 
that  protoplasm  can  manufacture  living  substance  out  of  the 
lifeless  and  add  the  same  to  itself.  It  can  replace  with  new 
and  fresh  living  matter  the  waste  material  which  is  used  up 
or  discarded  during  the  life  processes. 


168         THE  LOWEST  ORGANISMS  AXD  THE   CELL 

Protoplasm  lias  the  power  of  reproduction.  Reproduction  ac- 
companies growth  and  depends  upon  cell  division.  Of  course 
the  surface  area  of  a  cell  cannot  increase  in  the  same  ratio  as 
its  bulk.  The  surface  is  the  region  of  the  cell  through  which 
some  of  the  most  important  life  processes  of  assimilation  and 
respiration  take  place,  and  a  certain  amount  of  cell  surface  is 
necessary  for  a  given  bulk  of  protoplasm.  Therefore  the  cell 
divides  when,  after  a  period  of  growth,  the  bulk  of  protoplasm 
becomes  proportionally  too  great  for  the  amount  of  surface  area. 
The  sum  total  of  the  surfaces  of  the  daughter  cells  is  materially 
increased  by  their  division,  while  the  combined  bulk  of  their 
protoplasm  remains  the  same  as  before. 

A  living  being  is  like  a  machine  in  that  it  requires  fuel  to 
generate  its  energy  or  power  of  doing  work,  but  the  organism 
has  the  peculiar  ability  of  making  its  own  repairs,  of  increas- 
ing in  size,  and  of  detaching  from  itself  portions  which  can  in 
their  turn  attain  the  structure  and  efficiency  of  the  parent.  The 
process  of  life  is  continuous,  although  the  material  of  protoplasm 
is  constantly  changing,  —  that  is,  substances  are  constantly 
going  into  the  organism  and  substances  are  going  out.  It  may 
be  compared  to  a  whirlpool  in  a  river:  the  form  and  action  of 
the  whirlpool  is  constant,  although  the  water  which  enters  and 
leaves  remains  for  only  a  short  time  in  circular  movement. 

Protoplasm  always  comes  from  preexisting  protoplasm.  This 
means  that  protoplasm,  so  far  as  we  know,  never  springs  into 
existence  from  inorganic  material.  It  is  never  formed  de  novo. 
There  have  been  naturalists  and  philosophers  who  believed  that 
life  might  arise  spontaneously  under  favorable  conditions  in 
suitable  nutrient  solutions.  They  cited  such  illustrations  as 
the  swarming  microscopic  life  which  appears  in  extracts  or  infu- 
sions of  animal  and  vegetable  matter  as  examples  of  sponta- 
neous generation.  These  theories  were  overthrown  chiefly  by 
the  work  of  Pasteur  and  Tyndall,  who  showed  that  life  never 
appears  in  these  extracts  and  infusions  provided  proper  care 
is  taken  to  kill  all  organisms  that  may  be  in  them,  together 


DISTINCTIONS  BETWEEN  ANIMALS  AND  PLANTS     169 

with  all  spores  or  other  reproductive  cells,  and  then  to  pre- 
vent the  entrance  of  any  more  germs.  An  experiment  in  this 
line  can  be  performed  by  heating  an  extract  in  a  flask,  closed 
by  a  plug  of  cotton,  until  all  germs  have  been  killed.  Such  a 
solution  then  will  not  even  produce  the  bacterial  growths  that 
cause  decomposition,  for  the  cotton  plug  prevents  the  entrance 
of  any  dust.  It  is  now  established  that  all  organisms  at  present 
on  the  earth  are  generated  only  by  their  like ;  that  life  only 
comes  from  life,  and  protoplasm  from  preexisting  protoplasm. 

202.  The  distinctions  between  animals  and  plants.    Plants 
in  general  are  distinguished  from  animals  by  two  important 
peculiarities. 

First.  The  presence  of  chlorophyll,  or  equivalent  pigments, 
enables  the  plant  to  manufacture  its  own  food  by  photosynthesis 
in  the  interior  of  its  own  cells.  Animals  require  foods  already 
manufactured  by  other  animals  or  plants,  and  this  food  is  ab- 
sorbed from  without  the  cell. 

Second.  Plants,  when  growing,  are  generally  stationary,  with 
a  firm,  widely  expanded,  rigid  structure,  while  animals  are  more 
rounded,  compact,  and  yielding.  These  differences  are  deter- 
mined by  the  fact  that  the  protoplasts  of  plants  are  inclosed 
in  cellulose  compartments.  The  cell  wall  gives  to  plant  struc- 
ture a  degree  of  stiffness  which  greatly  limits  or  almost  pre- 
vents movement,  but  the  individual  protoplasts  of  plants  have 
all  the  characteristics  of  life  in  common  with  animal  cells, — 
sensation,  movement,  and  the  powers  of  growth,  repair,  and 
multiplication. 

203.  Some  organisms  of  doubtful  position.    Several  groups 
of  lowly  organisms  have  characters  which  are  in  part  plant-like 
and  in  part  animal-like.    We  shall  consider  briefly  only  one  of 
these  groups,  the  flagellates.1  . 

1  Another  large  group  of  doubtful  position  is  the  slime  molds,  or 
Myxomycetes,  more  frequently  included  among  plants  than  among  ani- 
mals, but  too  special  for  this  account.  See  MacBride,  The  North  American 
Slime  Moulds,  1899. 


170 


THE  LOWEST  ORGANISMS  AND  THE   CELL 


204.  The  flagellates.1  The  flagellates  (Flagellata)  are  aquatic, 
motile  forms,  either  one-celled  or  consisting  of  colonies  of  cells 
held  together  in  a  common  gelatinous  secretion.  The  individual 


FIG.  172.  Two  flagellate  forms 

A,  Euglena,  the  motile  cell  shown  above,  with  its  cilium  and  pigment  spot  at  the 
forward  end,  the  process  of  reproduction  by  simple  division  while  in  a  resting 
condition  being  illustrated  below;  B,  Uroglena  Americana,  a  large  colonial 
flagellate.  —  B,  adapted  after  Moore 

cells  are  provided  with  one,  two,  or  sometimes  more  delicate 
hair-like  appendages  called  cilia  (singular,  cilium,  meaning  an 
eyelash),  which  move  rapidly  in  the  water  and  are  organs  of 
locomotion.  Some  forms  have  chromatophores  and  can  therefore 
manufacture  their  own  food,  while  others  are  colorless  and  take 
their  food  in  animal  fashion  through  a  funnel-like  depression  into 

1  The  best  account  of  the  flagellates  will  be  found  in  Engler  and  Prantl, 
Die  Naturlichen  Pflanzenfamilien. 


THE   FLAGELLATES  171 

the  interior  of  the  cell.  A  bright  red  pigment  spot,  frequently 
found  in  each  cell,  is  regarded  as  a  structure  sensitive  to  light, 
for  these  organisms  generally  move  towards  the  source  of  bright 
illumination.  The  flagellates  are  believed  to  be  related  to  the 
lowest  green  plants,  the  algae,  and  some  groups  of  algae  are 
thought  to  have  been  derived  from  them. 

Euglena  (Fig.  172,  A)  is  a  common  flagellate  found  in  stag- 
nant pools.  The  cells  are  generally  green,  but  some  of  the  spe- 
cies and  related  forms  are  colorless,  having  adopted  the  habit 
of  living  exclusively  on  organic  food  substances  in  the  drain- 
age water  which  they  frequent.  Euglena  gracilis  becomes  quite 
colorless  when  cultivated  in  solutions  of  sugar  away  from  the 
light,  thus  suggesting  the  way  in  which  colorless  plants,  such 
as  the  fungi,  may  have  arisen  from  chlorophyll-bearing  ances- 
tors under  an  environment  which  supplied  an  abundance  of 
organic  food. 

Uroglena  (Fig.  172,  B)  is  a  colonial  flagellate  which  frequently 
appears  during  the  summer  months  in  reservoirs  and  gives  a 
fishy,  oily  taste  and  odor  to  the  water,  making  it  unfit  for  use. 
The  taste  and  odor  are  caused  by  globules  of  oil  that  are  set 
free  by  the  rupture  of  the  delicate  cells  when  the  water  is 
carried  through  pipes.  This  is  one  of  the  organisms  which  can 
easily  be  destroyed  by  treating  reservoirs  with  copper  sulphate.1 

1  See  papers  by  Moore  and  Kellerman,  United  States  Department  of 
Agriculture,  Bureau  of  Plant  Industry,  Bulletin  64,  1904  ;.also  Bulletin  76, 
1905. 


CHAPTEK  XIX 
THE   THALLOPHYTES 

205.  The  thallophytes.  The  branch  Thallophyta  (meaning 
thallus  plants)  contains  the  lowest  forms  in  the  plant  kingdom. 
A  thallus  is  a  simple  vegetative  body,  without  stems,  leaves,  or 
roots,  in  the  usual  sense.  The  groups  of  the  thallophytes  fall 
naturally  into  two  series  known  as  algae  and  fungi. 

"The  algce.  The  algae  contain  chlorophyll  or  other  pigments 
which  can  do  the  work  of  photosynthesis. 

The  fungi.  The  fungi  have  no  chlorophyll,  and  must  there- 
fore obtain  their  food  either  as  parasites  from  the  tissues  of 
living  plants  or  animals,  called  their  hosts,  or  they  may  live  as 
saprophytes  (meaning  decay  plants)  upon  the  products  of  decay. 

The  fungi  are  believed  to  have  been  derived  from  algae  which 
lost  their  color  and  gave  up  the  processes  of  photosynthesis 
because  they  happened  to  be  placed  under  conditions  favorable 
to  a  life  of  saprophytism  or  parasitism.  A  perfect  classification 
of  the  thallophytes  should  show  the  relationships  of  the  fungi 
to  the  algae,  but  these  are  so  little  understood  that  it  seems 
best  for  the  present  to  treat  the  two  groups  separately. 

The  thallus  is  not  really  the  distinguishing  character  of  the 
thallophytes,  for  some  higher  plants,  as  the  liverworts,  have 
thalloid  plant  bodies,  and  some  of  the  algae  have  a  stem  and  leaf 
structure  as  complex  as  that  of  the  mosses.  The  thallophytes 
are  separated  from  the  next  higher  group,  the  bryophytes  (liver- 
worts and  mosses),  by  the  absence  of  a  peculiar  type  of  life 
history  characterized  by  certain  complicated  reproductive  organs. 
These  peculiarities  cannot  be  understood  until  the  liverworts 
and  mosses  have  been  studied,  so  a  full  definition  of  the  thallo- 
phytes will  be  deferred  until  the  end  of  Chapter  xxiv. 

172 


CHAPTEK  XX 
THE  ALG.2E,  THE  LOWEST  GREEN  PLANTS 

206.  The  algae.*  For  the  present  we  may  think  of  the  thallo- 
phytes  as  the  immense  assemblage  of  plants  below  the  liver- 
worts, mosses,  ferns,  and  seed  plants.  In  number  of  species  and 
divergent  evolutionary  lines  the  group  is  much  the  largest  of 
the  four  divisions  of  the  plant  kingdom  (Thallopliyta,  Bryophyta, 
Pteridopliyta,  and  Spermatophyta). 

The  algae  are  thallophytes  whose  plant  bodies  are  colored 
because  the  cells  contain  chromatophores.  Almost  all  of  the 
fresh-water  forms  are  green,  but  the  majority  of  the  marine 
algae,  or  seaweeds,  are  either  brown  or  of  beautiful  shades  of  red. 
The  green  color  is,  of  course,  due  to  chlorophyll,  while  the  brown 
and  red  tints  are  caused  by  other  pigments.  The  algse  are 
divided  into  four  classes  as  follows : 

Class  I.  The  blue-green  algae,  or  Cyanopliycece. 

Class  II.  The  green  algae,  or  Chlorophycece. 

Class  III.  The  brown  algae,  or  Phceophycece. 

Class  IV.  The  red  algae,  or  Rhodophycece. 

It  might  appear  from  the  above  that  the  algaB  are  classified  by 
their  color,  but  this  is  not  true.  These  four  groups  are  defined 
by  peculiarities  of  cell  structure,  life  history,  and  methods  of 
reproduction  which  can  only  be  understood  through  a  study  of 
types  in  the  laboratory,  and  the  summaries  of  these  characters 

*To  THE  INSTRUCTOR  :  This  chapter  describes  many  more  types  than  it 
would  be  desirable  to  present  in  a  general  course.  The  instructor  should 
make  selections  according  to  the  material  available  (which  varies  greatly  in 
different  sections  of  the  country),  and  the  time  at  the  disposal  of  the  class. 
A  brief  discussion  of  the  best  and  most  available  types,  and  the  reasons  why 
they  are  desirable  for  laboratory  work  will  be  found  in  the  laboratory 
manual  of  the  authors. 

173 


174 


THE   ALG^E 


must  follow  the  accounts  of  the  groups.  However,  it  is  an  inter- 
esting fact  that  representative  algre  of  these  four  classes  can 
generally  be  picked  out  at  a  glance  by  their  color  alone. 


CLASS  I.    THE   BLUE-GKEEN   ALG.E,   OE 
CYANOPHYCE^E 

207.  The  blue-green  algae.    The  simplest  types  of  plants  are 
found  among  the  blue-green  algae  and  in  that  related  group  of 

the  fungi  called 
the  bacteria. 
Some  of  these 
plants  are  the 
most  primitive 
forms  of  life 
now  present  on 
the  earth. 

.  i  /  /  //v.*'*ain//7.S'\\  \  \ 

A*3^A 


208.  The  one- 
celled  blue- 
green  algae. 

These   forms 
develop  as 


FIG.  173.   One-celled  blue-green  algae  and  their 
cell  colonies 


A,  Gloeocapsa,  solitary  cell  and  small  groups  held  together 
by  the  thick  gelatinous  envelopes ;  B,  Clathrocystis  serugi- 
nosa,  cell  colony  of  many  hundreds  of  protoplasts  im-  slimy  growths 
bedded  in  a  jelly-like  substance ;  x,  single  cells  illustrating  on  £jie  surface 
division  by  fission 

of  stones,  wood- 
work, and  other  objects,  but  certain  types  float  freely  in  the 
water  in  small  groups,  or  sometimes  in  large  cell  colonies.  The 
following  types  are  representative. 

Gloeocapsa1  (Fig.  173,  A)  consists  of  cells  with  peculiar  soft 
walls  which  form  concentric  envelopes  around  the  groups  of 
protoplasts.  It  is  evident  that  the  wall  of  each  protoplast  per- 
sists for  a  long  tune  after  every  cell  division,  so  that  groups  of 

1  Chroococcus  is  an  excellent  substitute  for  Gloeocapsa,  and  is  not  uncom- 
mon in  stagnant  pools  and  on  wet  clay  banks.  Its  cells  are  solitary  and  lack 
the  gelatinous  envelopes  of  Glxocapsa. 


THE  FILAMENTOUS  BLUE-GREEN  ALGJ5  175 

daughter,  granddaughter,  and  even  great-granddaughter  cells 
may  remain  inclosed  in  the  envelope  of  the  original  mother  cell, 
which  becomes  very  much  swollen  and  jelly-like.  The  outer 
walls  of  the  groups  of  cells  finally  become  changed  to  a  soft 
mucilage,  so  that  the  groups  of  Glceocapsa  cells  form  at  times 
slimy,  dark  green  patches  over  damp  earth,  rocks,  and  logs. 
The  individual  protoplasts  have  an  exceedingly  simple  structure, 
for  the  coloring  matter  is  uniformly  distributed  through  the 
cells  and  no  nucleus  can  be  seen. 

ClatJirocystis  and  Ccdosphcerium  are  free,  floating  cell  colonies, 
often  forming  greenish  scums  during  the  summer  months  on 
the  surface  of  park  ponds,  reservoirs,  and  other  small  bodies  of 
water.  The  colonies  of  Ccdospheerium  are  spherical,  while  those 
of  ClatJirocystis  (Fig.  173,  B)  become  irregular  in  shape  through 
the  development  of  holes,  so  that  the  structure  is  somewhat 
net-like. 

209.  The  filamentous  blue-green  algae.  These  frequently 
form  felted  or  tufted  growths  or  gelatinous  expansions  of  con- 
siderable size.  There  are  a  number  of  complex  branching  types, 
but  the  following  are  good  examples  of  the  assemblage. 

Oscillatoria  is  the  most  interesting  type  of  the  Cyanopliycece 
if  only  one  form  can  be  studied.  The  filaments  are  generally 
made  up  of  flattened  disk-shaped  cells,  placed  face  to  face  within 
an  exceedingly  delicate  sheath,  much  like  a  roll  of  coins  wrapped 
in  paper.  Cell  division  takes  place  in  all  portions  of  the  fila- 
ment, and  several  stages  are  illustrated  in  Fig.  174,  A.  Growth 
is  therefore  not  confined  to  the  tip  or  any  other  special  region 
of  the  plant.  New  filaments  arise  by  the  breaking  apart  of  the 
older  ones,  generally  at  some  point  where  one  or  more  cells,  have 
died  (Fig.  174,  A,  d).  The  end  cells  of  filaments  or  fragments  of 
filaments  are  always  rounded,  illustrating  beautifully  the  phe- 
nomenon of  cell  turgor  or  pressure  from  within  the  protoplast 
upon  the  cell  membrane.  The  cell  structure  of  Oscillatoria  is 
very  typical  of  the  blue-green  algae.  The  blue-green  pigment 
gives  color  to  the  entire  outer  region  of  the  protoplast,  which 


176 


THE  ALG^E 


may  be  considered  a  diffused  chromatophore.  There  is  no  nucleus 
in  the  usual  sense  of  the  term,  although  the  central  region  of  the 
cell  has  a  different  structure  from  the  outer  and  probably  contains 
chromatin.  The  small  granules  arranged  along  the  cross  walls 
are  believed  to  be  food  products  built  up  by  the  activities  of 
the  blue-green  pigment  in  sunlight  (photosynthesis).  Oscillato- 

ria  takes  its  name  from 
the  remarkable  move- 
ments of  the  filaments, 
whose  free  ends  swing 
back  and  forth  describ- 
ing a  circle  or  an  ellipse, 

h      ^^^~        mi         while  the  filaments  may 

glide  slowly  forward. 
The  cause  of  this  move- 
ment is  not  understood. 
Oscillator ia  is  found  in 
greatest  abundance  in 
open  drains,  ditches,  or 
pools,  where  the  water 
is  foul  with  decaying 
There 

it  may  form  thick  felts 
on  the  bottom,  or  rise 
to  the  surface  in  slimy 
masses  because  of  the 
bubbles  of  gas,  largely 
oxygen,  formed  during  the  processes  of  photosynthesis  and  held 
within  the  tangle  of  filaments. 

Anabcena  and  Nostoc  are  closely  related  genera.  The  fila- 
ments are  chains  of  round  or  elliptical  cells.  Besides  the  blue- 
green  vegetative  cells  there  are  present  at  intervals  curious  cells 
termed  heter ocysts  (meaning  other  cells),  which  are  generally 
larger  than  the  vegetative  cells,  lighter  in  color,  and  often  empty 
of  protoplasm.  Their  function  is  not  clearly  understood.  The 


FIG.  174.  Filamentous  blue-green  algae 

A,  Oscillatoria ;  d,  dead  cell,  indicating  a  point  organic  matter, 
where  the  filament  might  break  apart ;  f,  stages 
of  cell  fission;  B,  Anabsena;  h,  heterocyst; 
s,  resting  cells ;  C,  Nostoc,  habit  sketch  of  a  col- 
ony and  the  details  of  a  single  filament ;  h,  heter- 
ocyst; D,  Gloeotrichia,  portion  of  a  colony  and 
the  base  of  a  single  filament  in  detail;  h,  heter- 
ocyst ;  s,  resting  cell 


LIFE  HABITS  OF   THE  BLUE-GREEN  ALG^E         177 

filaments  may  break  apart  on  either  side  of  the  heterocyst,  set- 
ting free  chains  of  cells  which  grow  into  new  filaments.  Certain 
vegetative  cells  in  Anabcena  increase  greatly  in  size  and  become 
densely  filled  with  protoplasm  and  food  material  and  sur- 
rounded by  a  thick  protective  wall  (Fig.  174,  B,  s).  Such  cells 
are  called  resting  cells,  or  spores,  and  they  are  able  to  live  through 
seasons  of  drought  or  a  winter's  cold  and  with  the  return  of 
favorable  conditions  to  germinate  and  form  new  filaments.  The 
filaments  of  Anabcena  are  held  in  a  soft  slime,  but  those  of  Nostoc 
are  surrounded  by  a  stiff  jelly,  so  that  the  mass  of  much-coiled 
chains  of  cells  has  a  firm  boundary.  Consequently,  Nostoc  colo- 
nies (Fig.  174,  C)  may  have  a  spherical  form  and  become  as 
large  as  marbles.  The  slimy  or  jelly-like 'substance  of  Anabcena 
and  Nostoc  is  a  modification  of  the  delicate  sheath  around  the 
filaments  and  corresponds  to  the  envelopes  about  the  cells  of 
Grlceocapsa. 

Glceotricliia  (Fig.  174,  D)  sometimes  develops  in  such  quan- 
tities in  ponds  and  lakes  during  the  summer  as  to  form  a 
brilliant  green  scum  on  the  surface  of  the  water.  The  fila- 
ments have  a  radiate  arrangement  in  a  soft,  gelatinous  sub- 
stance and  end  in  long  hairs,  and  a  very  large  resting  cell  may 
be  formed  at  the  base  of  each  filament  adjacent  to  the  terminal 
heterocyst. 

210.  Life  habits  of  the  blue-green  algae.  The  Cyanophycece 
have  some  peculiar  life  habits  of  ecological  interest.  They  are 
generally  found  in  warmish  waters,  both  fresh  and  salt,  and 
many  of  the  forms  prefer  those  which  are  foul  with  decaying 
organic  matter.  Thus  open  drains  and  reeking  pools  of  stagnant 
water  present  luxuriant  growths  of  these  algae.  It  is  probable 
that  the  plants  actually  use  for  food  certain  of  the  organic  sub- 
stances in  such  waters.  Some  of  the  most  conspicuous  green 
scums  on  ponds  and  small  lakes  are  composed  of  certain  of  these 
algse  (Ccelo  splicer  ium,  Clathrocystis,  Anabcena,  and  Glceotrichia f 
etc.).  Such  scums  may  be  called  water  blooms,  after  the  German 
term  Wasserbliithe.  The  coloration  of  the  Red  Sea  is  due  to  an 


178  THE  ALG^E 

extensive  water  bloom  caused  by  a  filamentous  blue-green  alga 
(Trichodesmium)  which  at  times  fills  the  water,  and  whose  color, 
a  reddish  brown,  gives  then  a  peculiar  tint  to  the  sea. 

Some  forms  (Anabcena,  Clatlirocystis,  and  certain  species  of 
Oscillatoria)  are  frequently  responsible  for  the  fouling  of  water 
supplies  which  take  on  what  is  called  the  "pigpen  odor,"  and 
are  otherwise  unfit  for  public  use.  All  of  these  blue-green  algae, 
together  with  the  flagellate  Uroglcna,  can  be  killed  by  treating 
the  reservoir  or  other  body  of  water  with  copper  sulphate  (see 
Sec.  204),  a  perfectly  safe  and  inexpensive  remedy  for  contami- 
nated water  supplies. 

Perhaps  the  most  remarkable  display  of  the  blue-green  algae 
is  in  the  waters  of  certain  hot  springs,  as  in  Yellowstone 
National  Park.  It  is  doubtful  whether  any  algae  except  the 
Cyanophyccce  can  live  in  water  warmer  than  100°  F.  (40°  C.), 
but  some  of  the  blue-green  algae  grow  luxuriantly  in  hot  springs 
at  137°-145°  F.  (58°-63°  C.).  It  is  probable  that  their  simple 
cell  structure  makes  possible  a  greater  power  of  resistance  to 
these  extraordinary  life  conditions. 

211.  Summary  of  the  blue-green  algae.  The  Cyanophycea 
are  distinguished  from  other  groups  of  algae  by  the  simpli- 
city of  their  cell  structure,  the  absence  of  sexual  reproduction, 
and  the  presence  of  a  blue-green  pigment  uniformly  diffused 
through  the  outer  region  of  the  cells.  The  method  of  growth 
by  rapid  divisions  or  splitting  of  the  cells  throughout  the  entire 
plant  body  is  a  very  characteristic  feature  of  the  group,  and 
the  blue-green  algae  are  sometimes  called  the  "fission  algae" 
(Schizophycece). 

The  blue-green  algae  agree  with  the  bacteria,  or  "fission  fungi" 
(Scliizomycetes),  in  their  simplicity  of  cell  structure  and  methods 
of  reproduction,  but  the  bacteria  are  of  course  generally  without 
pigment.  It  is  quite  clear  that  the  Cyanopliycece  and  the  Scliizo- 
mycetes  are  closely  related,  and  some  authors  place  them  together 
in  a  separate  division  of  the  plant  kingdom  below  the  Thallo- 
phyta,  named  the  Schizophyta,  or  "  fission  plants." 


PLEUROCOCCUS  179 

CLASS  II.    THE  GREEN  ALGLE,  OK   CHLOROPHYCE^ 

212.  The  green  algae.    The  green  algae  comprise  a  large  and 
varied  assemblage  of  groups,  many  of  which  are  widely  different 
from  one  another.    Some  forms  of  the  Chlorophycece  are  believed 
to  stand  rather  close  to  what  was  the  main  line  of  ascent  from 
the  algie  to  the  liverworts  and  mosses.     Consequently  the  class 
has  an  important  place  in  an  account  of  the  evolution  of  the 
plant  kingdom.    The  green  algse  illustrate  better  than  any  other 
group  the  origin  and  evolution  of  sexual  processes  in  plants. 
Since  the  more  familiar  algal  growths  of  fresh  water  are  green 
algse,  a  more  extended  treatment  of  the  Chloropliycece  will  be 
given  than  of  the  less  familiar   groups  of    the    Cyanopliycece, 
Phceopliycece,  and  Rhodophycece,  and  the  following  six  orders  will 
be  considered  together  with  a  "  Summary  of  the  Green  Algse." 

Order  1.  The  one-celled  green  algse,  or  Protococcales. 

Order  2.  The  confervas,  or  Confervales. 

Order  3.  The  pond  scums  and  desmids,  or  Conjugates. 

Order  4.  The  diatoms,  or  Diatomales. 

Order  5.  The  siphon  algse,  or  Siphonales. 

Order  6.  The  stone  worts,  or  Charales. 

ORDER  1.  THE  ONE-CELLED  GREEN  ALG^E,  OR  PROTOCOCCALES 

213.  The  one-celled  green  algae.    This  order  contains  almost 
all  of  the  one-celled  green  algse  excepting  the  large  but  very 
special  groups  of   the    desmids    and    diatoms.    We    can  only 
describe  briefly  five  types. 

214.  Pleurococcus.   Pleurococcus  (family  Pleurococcacece)  forms 
the  green  coating  or  stain  that  is  very  common  on  the  north 
sides  of  tree  trunks,  fences,  and  stone  walls.    The  cells  (Fig.  175) 
may  be  solitary,  but  they  usually  remain  associated  in  small 
groups  for  some  time  after  the  cell  divisions.    The  protoplast 
contains  generally  a  single  chromatophore  of  irregular  shape 
which,  as  a  rule,  fills  the  greater  part  of  the  cell.    The  chroma- 
tophore is,  however,  variable  in  size  and  may  or  may  not  have 


180 


THE  ALG^E 


FIG.  175.   Pleurococcus  vul- 
garis,  a  one-celled  green 


Several  cells  illustrating  the 
method  of  cell  division  and 
their  association  in  small 
groups 


a  pyrenoid  (Sec.  196).  The  nucleus  can  sometimes  be  seen  in 
the  center  of  the  cell.  The  cells  are  exceedingly  resistant  to 
cold  and  drought,  but  under  very  severe  conditions  they  protect 
themselves  by  forming  a  heavy  cell  wall,  thus  becoming  resting 
cells.  Sometimes  the  contents  of  a  cell 
break  up  into  several  daughter  proto- 
plasts, but  as  a  rule  the  only  method 
of  reproduction  is  by  simple  cell  divi- 
sion. Other  forms  of  one-celled  algae, 
with  more  complicated  methods  of  re- 
production (by  zoospores  and  gametes), 
are  frequently  found  associated  with 
Pleurococcus,  but  should  be  carefully 
distinguished  from  this  simple  alga. 

Pleurococcus  may  seem  almost  as 
simple  as  the  one-celled  blue-green  alga 
Gloeocapsa,  but  its  cell  structure  with 
a  chromatophore  and  well-defined  nucleus  is  far  in  advance  of 
the  Cyanopliycece. 

215.  Sphaerella  and  Volvox.*  These  forms  are  representatives 
of  one  of  the  most  interesting  families  of  the  green  algse,  the 
Volvox  family1  (Volvocacece).  The  lowest  members  are  one- 
celled  and  resemble  the  flagellates  (Sec.  204),  but  the  higher 
forms  are  cell  colonies  of  remarkable  structure  and  life  histories. 
The  vegetative  cells  are  motile,  being  always  provided  with  two 

*  To  THE  INSTRUCTOR  :  It  is  rather  difficult  to  obtain  material  of  the 
Volvox  family,  and  it  cannot  be  depended  upon  for  type  study.  Therefore 
laboratory  work  on  the  reproductive  processes  in  the  algse  can  much  better 
be  arranged  with  such  types  as  Ulothrix,  or  Ulva,  or  some  form  of  the  Chce- 
tophoracece,  or  Cladophora,  CEdogonium,  or  Vaucheria  and  Fucus.  But  the 
Volvocacece  and  Flagellata  are  so  important  to  a  conception  of  certain  primi- 
tive conditions  of  algal  life  that  they  should  be  discussed  in  any  extended 
general  course.  The  fact  that  zoologists  have  found  Volvox  and  its  relatives 
of  interest  should  not  deter  botanists  from  making  use  of  their  own. 

1  For  a  detailed  account  of  the  Volvocacece  see  Goebel,  Outlines  of  Classi- 
fication and  Special  Morphology  of  Plants,  and  Engler  and  Prantl,  Die  Natiir- 
lichen  Pflanzenfamilien. 


SPHSERELLA 


181 


hair-like  cilia,  whose  incessant  whipping  of  the  water  carries  the 
organism  along.  There  is  also  a  red  pigment  spot  at  the  ciliated 
end  of  the  cell  (Fig.  176,  A,  p).  This  free-swimming,  ciliated 
cell  is  of  a  type  strikingly  different  from  Pleurococcus,  but  it 
is  believed  to  represent  very  nearly  the  ancestral  condition  of 
many  groups  of  algse. 

Sphcerella  lacustris  (Hcematococcus  pluvialis)  is  found  freely 
swimming  in  rock  pools  and  sometimes  in  troughs  and  basins, 


FIG.  176.    Sphcerella 

A,  />,  Sphserella  lacustris:  a  single  cell  in  detail  and  a  group  of  daughter  proto- 
plasts within  the  parent  cell.  C,  D,  Sphserella  Butschlii:  numerous  small  sex- 
ual elements  or  gametes  are  shown  in  the  parent  cell  C,  and  D  illustrates  their 
fusion  in  pairs  to  give  the  sexually  formed  cell  or  zygospore  z.  —  Z>,  after 
Schenck ;  C,  I),  after  Blochmaun 

and  is  frequently  so  abundant  as  to  color  the  water  a  bright 
green.  The  organism  multiplies  very  rapidly,  for  the  larger  indi- 
viduals (Fig.  176,  A)  form  2-16  daughter  cells  (Fig.  176,  B), 
which  escape  from  the  mother-cell  membrane,  swim  away,  and 
after  a  period  of  growth  form  in  their  turn  a  new  set  of 
daughter  cells.  The  free-swimming  cells  come  to  rest  at  times, 
becoming  thick-walled  resting  cells,  which  are  colored  red  by 
a  peculiar  pigment.  These  resting  cells  carry  the  plant  over 


182 


THE  ALG^E 


unfavorable  seasons  and  are  sometimes  developed  so  numerously 
as  to  cover  the  bottom  of  pools  and  rock  hollows  with  a  red 
deposit.  The  phenomenon  called  "  red  snow  "  is  due  to  deposits 
of  the  resting  cells  of  Sphcerella  nivalis  on  fields  of  snow  and  ice. 
Some  species  of  Sphcerella  (Fig.  176,  C)  develop  a  much  greater 
number  of  daughter  elements,  32  to  64,  which  are  much  smaller 

than  those  just  described,  but 
have  the  same  structure.  These 
smaller  cells  swim  about  freely 
for  a  short  time,  and  then  come 
together  in  pairs  and  completely 
fuse  with  one  another,  beginning 
at  the  ciliated  ends  (Fig.  176,  D). 
A  cell  fusion  of  this  character  is  a 
sexual  act  (Sec.  200)  and  the  cells 
which  unite  are  gametes.  The  sex- 
ually formed  fusion  cell  or  zyg- 
ospore  of  Sphcerdla  soon  settles 
down  on  some  surface  and,  losing- 
its  four  cilia,  remains  quiet  for 
several  days  or  weeks,  finally  de- 
veloping within  itself  several 
motile  cells  of  the  usual  type. 

Volvox  (Fig.  178,  A)  is  a  colo- 
nial form  consisting  of  many  hun- 
dreds of  cells  (sometimes  more 
than  twenty  thousand)  imbedded 
in  a  gelatinous  substance  in  the 
form  of  a  sphere,  with  the  pairs  of  cilia  pointing  outwards. 
These  remarkable  organisms,  as  large  as  pin  heads,  roll  slowly 
through  the  water  of  quiet  pools  and  ponds,  sometimes  gather- 
ing in  great  numbers  in  open  sunlit  portions,  free  from  water 
weeds  and  algal  growths.  Daughter  colonies  (Fig.  178,  A,  d)  are 
formed  from  certain  cells  which  after  a  period  of  growth  develop 
a  large  number  of  motile  cells  like  the  parent.  These  small  cells 


EIG.  177.    Chlamydomonas 
Braunii 

Chlamydomonas  is  not  uncommon 
in  the  same  sort  of  situations  as 
Sphserella.  It  may  be  distinguished 
from  the  latter  by  the  absence  of 
a  thick  gelatinous  envelope  around 
the  cells.  Some  of  the  forms  show 
important  advances  over  Sphse- 
rella in  their  sexual  processes,  for 
the  gametes  may  be  of  two  sizes, 
large  female  and  small  male  cells, 
as  shown  above,  z,  the  zygospore. 
—  After  Goroschankin 


VOLVOX 


183 


become  arranged  to  form  a  daughter  colony  which  swims  around 
in  the  interior  of  the  mother  colony.  Sometimes  several  of  the 
daughter  colonies  may  be  developed,  and  they  finally  escape 
by  the  rupture  of  the  parent  structure.  The  sexual  cells,  or 
gametes,  are  of  two  sorts  :  (1)  large  female  cells,  which  are  called 
eggs  because  they  are  without  cilia  and  consequently  never 
motile,  and  (2)  small  male  cells,  or  sperms,  of  peculiar  form, 
with  two  cilia,  and  consequently  very  actively  motile.  The  eggs 


FIG.  178.    Volvox  globator,  a  colonial  form  of  the  Volvocacece 

A,  mature  colony,  with  four  daughter  colonies  developing  in  its  interior;  B,  sec- 
tion of  the  edge  of  the  colony,  showing  three  vegetative  cells  and  a  developing 
egg;  C,  a  packet  of  sperms  within  the  parent  cell  and  a  single  sperm  very 
much  magnified  at  the  side;  D,  an  egg  surrounded  by  a  swarm  of  sperms; 
E,  an  oospore  with  heavy  protective  wall.  —  C,  after  Colin. 

(Fig.  178,  B,  Z>),  formed  by  the  enlargement  of  vegetative  cells, 
escape  into  the  interior  of  the  colony  as  naked  spherical  proto- 
plasts. The  sperms  (Fig.  178,  C)  are  developed  in  great  num- 
bers within  enlarged  vegetative  cells.  They  are  also  set  free 
within  the  parent  colony  and  gather  about  the  eggs  in  swarms 
(Fig.  178,  D).  Finally,  a  single  sperm  fuses  with  each  egg,  which 
is  then  said  to  be  fertilized.  The  fertilized  egg  immediately 
forms  a  cell  wall  about  itself  and  passes  through  a  period  of 
rest  as  an  oospore  (Sec.  200). 


184 


THE 


Volvox  thus  presents  a  great  advance  over  Sphoerella, 
Chlamydomonas,  and  other  one-celled  members  of  the  Volvo- 
cacece,  in  the  highly  developed  sexual  process  as  well  as  in  the 

complex  cell  colony.  There  is,  how- 
ever, a  series  of  genera  (G-onium,  Pan- 
dorina,  Eudorina,  Pleodorina,  etc.)  in 
the  family,  illustrating  intermediate 
conditions  between  these  extreme 
forms,  which  makes  it  clear  that  Vol- 
vox stands  at  the  head  of  a  remark- 
able line  of  development  in  the  algae. 
It  may  be  considered  the  climax  type 
of  a  side  line  of  evolution,  —  that  is, 
a  branch  which  departs  widely  from 
the  main  line  of  ascent. 


ORDER  2.  THE  CONFERVAS,  OR 

CONFERVALES 


216.  The  confervas.  The  Confer- 
vales  comprise  many  very  common 
filamentous  algae  and  also  such  mem- 
branous forms  as  the  sea  lettuce.  The 
algae  which  seem  to  be  nearest  to  the 
main  line  of  ascent  to  the  liverworts 
and  mosses  are  found  in  this  group. 
Some  of  the  types  illustrate  espe- 
cially well  the  principal  forms  of  sex- 
ual reproduction  in  the  algae  and 
various  types  of  life  histories. 
217.  Ulothrix.  This  confervoid  alga  (family  Ulothricacece) 
is  abundant  on  stones  and  rocks  along  the  shores  of  the  great 
lakes,  in  quieter  waters  at  the  seaside,  and  frequently  grows  in 
stone  fountains  or  on  stonework  around  park  ponds.  The  fila- 
ments are  unbranched,  and  each  consists  of  a  row  of  similar 


FIG.  179.   The  water  net 
(Hydrodictyon] 

This  is  a  remarkable  form  of 
the  Protococcales,  whose  cells 
form  the  meshes  of  a  net-like 
cell  colony,  A.  New  nets  are 
formed  in  the  interior  of  large 
cells,  B,  which  develop  an 
immense  number  of  zoospores 
that  never  escape  from  the 
mother  cell,  but  join  with  one 
another  to  form  daughter 
nets,  which  are  set  free  by  the 
breaking  down  of  the  mother- 

'  cell  wall 


ULOTHRIX 


185 


cells.  Each  cell  contains  a  single  chromatophore  with  pyrenoids, 
which  has  the  form  of  a  wide  band,  or  girdle,  just  under  the 
cell  wall,  and  generally  surrounds  the  nucleus  in  the  middle 
region  of  the  cell  (Fig.  180,  A,  B).  The  filaments  are  attached 
at  one  end  (Fig.  180,  A),  and  the  growth  by  cell  division  takes 
place  throughout  the  entire  length  and  is  not  confined  to  the 


FIG.  180.    Ulothrix  zonata 

A,  base  of  filament,  showing  its  attachment  and  cells  containing  band-shaped 
chromatophores  with  pyrenoids;  B,  portion  of  a  filament  about  fifty  cells 
above  the  base,  showing  a  vegetative  cell  below  and  two  cells  which  have 
formed  4  and  8  zoospores  respectively ;  C,  the  zoospores,  each  with  a  pigment 
spot  and  four  cilia;  D,  stages  in  the  germination  of  the  zoospore;  E,  portion 
of  a  filament  illustrating  the  formation  of  gametes,  64  in  each  cell;  F,  the 
gametes,  showing  pigment  spot  and  two  cilia,  and  stages  in  their  fusion  to  form  a 
four-ciliate  zygospore  with  two  pigment  spots ;  G,  germination  of  the  zygospore, 
which  develops  a  number  of  zoospores.  —  G,  after  Dodel 

tip  as  in  some  algae.  The  cells  in  the  upper  portions  of  older 
filaments  (Fig.  180,  B}  develop  a  type  of  reproductive  cell  very 
common  among  the  algae,  called  the  zoospore  (meaning  animal 
spore)  because  of  its  animal-like  habit  of  swimming  about. 

Zoospores  are  naked  ciliated  protoplasts  formed  within  parent 
cells  called  sporangia.  They  swim  rapidly  through  the  water, 
and  each  generally  contains  a  red  pigment  spot.  Zoospores  are 


186  THE  ALG^E 

attracted  by  light  and  collect  at  the  illuminated  side  of  a  vessel, 
forming  a  green  cloud  in  the  water.  Because  of  these  habits, 
and  their  rapid  darting  to  and  fro  in  the  water,  they  are  often 
called  swarm  spores. 

The  zob'spores  of  Ulothrix  are  developed  most  numerously  at 
night  and  are  set  free  from  the  parent  filaments  chiefly  during 
the  morning  hours.  Sometimes  the  entire  protoplast  slips  out  as 
a  single  large  zoospore,  but  more  often  2,  4,  or  8  zoospores  are 
formed  in  each  parent  cell  (Fig.  180,  B).  They  are  roundish  or 
pear-shaped  (Fig.  180,  (7),  with  four  cilia  at  the  pointed  end,  and 
each  contains  a  red  pigment  spot,  chromatophore,  and  nucleus. 
The  zoospores  thus  resemble  the  organisms  called  flagellates 
(Sec.  204),  and  like  them  swim  freely  around  in  the  water  by 
the  lashing  movements  of  their  cilia.  But  the  zoospores  have 
a  relatively  short  free-swimming  period,  for  after  perhaps  an  hour 
or  more  they  attach  themselves  by  the  ciliated  ends  to  various 
objects  and  grow  into  new  Ulothrix  filaments  (Fig.  180,  D). 

At  times  a  much  greater  number  of  zoospores  may  be  de- 
veloped in  the  parent  cells,  —  perhaps  32 -or  64,  or  even  more 
than  a  hundred  (Fig.  180,  7?) .  These  have  generally  only  two 
cilia  and  are  much  smaller  than  the  four-ciliate  zoospores,  but 
otherwise  have  the  same  structure.  They  swim  very  actively 
for  a  short  time,  and  then  come  together  in  pairs  in  the  water 
and  fuse  with  one  another  (Fig.  180,  F).  This  cell  union  is  a 
sexual  process  (see  Sec.  200),  and  the  small  two-ciliate  zoospores 
are  therefore  sexual  elements  and  are  called  gametes.  The  result 
of  this  fusion  is  a  sexually  formed  cell  called  a  zygospore 
(meaning  a  yoked  spore),  because  the  two  gametes  are  similar, 
like  the  halves  of  a  yoke,  and  not  different  in  form,  as  the  sperm 
and  egg  (see  Fig.  178,  Volvox).  This  simple  type  of  sexual 
reproduction  is  termed  isogamy,  because  the  gametes  have  the 
same  form,  or  morphology.  The  zygospore,  of  course,  corresponds 
to  the  fertilized  egg,  or  oospore,  characteristic  of  higher  plants. 

The  zygospore  of  Ulothrix  swims  about  for  a  short  time  with 
its  four  cilia,  and  may  only  be  distinguished  from  the  large 


THE  ORIGIN  OF  SEX  IN  PLANTS 


187 


four-ciliate  asexual  zoospores  by  its  two  pigment  spots.  It  finally 
comes  to  rest  and  remains  quiet  for  many  weeks  or  several 
months,  but  slowly  increases  in  size.  Finally,  the  zygospore 
develops  several  zoospores  (Fig.  180,  G),  which  escape  from  the 
cell,  swim  off,  and  develop  new  Ulotlirix  plants. 

218.  The  sea  lettuces.    The  sea  lettuces  include    Ulva  and 
its  relatives  (family  Ulvacece),  and  are  very  common  along  the 
seacoast,  forming  green 

fringes  on  the  rocks  and 
wharves  near  low  water- 
mark. The  thallus  is  a 
thin  green  membrane  (Fig. 
181,^4)  instead  of  a  fila- 
ment as  in  Ulotlirix.  Zoo- 
spores  and  gametes  (Fig. 
181,  B,  C)  are  developed  in 
the  cells  along  the  edge 
of  the  membranes.  Their 
structure,  methods  of 
formation,  and  habits  are 
essentially  the  same  as  in 
Ulothrix,  and  the  sea  let- 
tuces are  equally  good  for 
the  study  of  these  points, 
and  they  are  sometimes 
more  available  than  Ulo- 
tlirix for  those  living  at  or 
near  the  seacoast. 

219.  The  origin  of  sex  in  plants.     Ulotlirix,  Ulva,  and  some 
other  types  show  clearly  that  the  simplest  forms  of  gametes 
in  plants  are  closely  related  to  zoospores,  for  they  are  devel- 
oped in  the  same  way  and  have  a  similar  structure.    Indeed, 
the  gametes  of  these  lower  plants  frequently  germinate  directly 
like  zoospores,  thus  showing  that  the  sexual  habit  of  fusing 
with  one  another  is  not  firmly  fixed.    However,  the  plants  that 


FIG.  181.  The  sea  lettuce  (Ulva) 

A,  habit  sketch;  B,  cells  forming  four-ciliate 
zoospores ;  C,  two-ciliate  gametes. —  Adapted 
after  Thuret 


188  THE 

develop  from  such  gametes  are  generally  smaller  and  weaker 
than  those  which  come  from  the  usual  zoospores.  For  these 
reasons  it  seems  evident  that  the  gametes  of  plants  arose  from 
zoospores,  or  motile  cells  similar  to  zoospores,  which,  adopting 
the  habits  of  fusing  in  pairs,  became  sexual  cells.  Such  types 
as  Ulotlirix  and  Ulva  have  an  especial  interest  because  they 
illustrate  the  general  conditions  which  must  have  been  present 
with  the  origin  of  sex  in  any  group  of  plants. 

220.  (Edogonium.  (Edogonium  (family  (Edogoniacece)  is  one 
of  the  best  illustrations  in  the  green  algae  of  the  higher  sexual 
condition  where  the  gametes  become  differentiated  and  specialized 
as  eggs  and  sperms. 

The  species  are  unbranched,  filamentous,  fresh-water  forms, 
attached  by  a  disk-like  development  from  the  lowest  cell  (Fig. 
182,  A)  called  a  holdfast.  The  cells  have  large  chromatophores 
of  irregular  form,  containing  pyrenoids.  There  are  sets  of  curi- 
ous lines  called  caps  across  the  ends  of  many  of  the  cells 
(Fig.  182,  B,  c),  —  structures  peculiar  to  this  family,  —  which 
result  from  a  method  of  cell  division  too  complicated  to  be 
described  here.1  Zoospores  are  developed  singly  in  the  cells, 
and  are  large  protoplasts  with  a  circle  of  cilia  at  a  colorless  end 
(Fig.  182,  Z>).-  After  swimming  about  for  a  short  time  the 
zoospores  settle  down  on  the  ciliated  end,  develop  the  hold- 
fasts, and  grow  at  once  into  new  filaments. 

The  sexual  cells,  or  gametes,  of  (Edogonium  are  eggs  and  sperms. 
The  eggs  are  developed  singly  in  enlarged  cells,  which  are  the 
female  sexual  organs  (Fig.  182,  B]  and  are  called  oogonia 
(singular,  oogonium).  The  entire  protoplast  of  the  oogonium 
becomes  the  egg  (Fig.  182,  B,  e),  which  remains  within  the  oogo- 
nium as  a  naked,  motionless  cell,  without  cilia,  and  is  richly 
supplied  with  chromatophores  and  food  material.  The  sperms 
are  developed  in  pairs  in  short,  disk-shaped  cells,  which  are 
found  in  small  groups,  forming  the  male  sexual  organ,  or 

1  See  Goebel,  Outlines  of  Classification  and  Special  Morphology  of  Plants, 
p.  44. 


(EDOGONIUM 


189 


antheridium  (Fig.  182, 
B,  a).  The  sperms,  fre- 
quently called  anthero- 
zoids  by  botanists,  are 
small,  almost  colorless 
protoplasts,  with  a  cir- 
cle of  cilia  at  one  end 
(Fig.  182,  C)  like  the 
zoospore.  They  are  in 
sharp  contrast  to  the 
eggs,  being  actively  mo- 
tile, ciliated,  and  with 
very  much  reduced 
chromatophore  and  food 
contents. 

A  cleft,  or  pore  (Fig. 
182,  B,  e),  is  formed  in 
the  oogonium  so  that 
the  sperms  may  enter, 
and  one  of  them,  fusing 
with  the  egg,  fertilizes 
it.  The  egg  after  fertili- 
zation develops  a  heavy 
wall  (Fig.  182,  jB,  o) 
and  becomes  an  oospore 
(meaning  an  egg  spore). 
The  obspores,  thus  pro- 
tected, can  live  through 
drought  or  winter's  cold 
and  so  survive  seasons 
of  the  year  impossible 
for  vegetative  growth. 
On  the  return  of  favor- 
able conditions  they 
germinate,  each  oospore 


FIG.  182.    (Edogoniiim  nodulosum 

A,  base  of  filament  showing  holdfast;  B,  filaments 
with  oogonia  and  antheridia ;  e,  an  egg  ready 
for  fertilization,  showing  the  cleft  for  the  en- 
trance of  the  sperm ;  o,  the  thick-walled  oospore ; 
a,  antheridium,  composed  of  four  cells ;  c,  caps ; 
C,  sperms,  showing  crown  of  cilia ;  I),  zoospore ; 
E,  germination  of  the  oospore,  producing  four 
zoospores. —  C,  D,  E,  after  Juranyi 


190 


THE  ALG^E 


giving  rise  to  four  large  zoospores  (Fig.  182,  E),  which  develop 
at  once  into  (Edogonium  plants. 

Both  eggs  and  sperms  are  believed  to  have  been  derived  from 
simpler  ancestral  types  of  ciliated  gametes,  similar  in  structure 
to  one  another  and  to  the  zoospore.  These  ancestral  sexual 
conditions  must  have  been  those  of  isogamy,  somewhat  as  is 
at  present  illustrated  in  Ulothrix.  The  originally  similar 
ciliated  gametes  varied  in  size.  The  smaller  reduced 
their  chromatophore  and  food  contents  because  they 
were  formed  in  large  numbers  but  retained  their  cilia 
and  thus  became  the  small  active  sperms.  The  larger 
gametes  accumulated  rich  supplies  of  food,  became  slug- 
gish, finally  lost  their  cilia  and  swim- 
ming habits,  and  at  last  were  retained 
within  and  protected  by  the  oogonia, 
thus  becoming  large  nonmotile  eggs. 

It  is  clear  that  the  plant  gains  very 
much  by  differentiating  and  specializ- 
ing its  gametes  as  eggs  and  sperms,  for 
the  eggs  are  protected  and  richly  sup- 
plied with  protoplasm  and  food,  while 
the  sperms  are  developed  very  numer- 
ously and  are  well  adapted  to  swim  FlG-  m  Draparnaldia,  one 
actively  about  in  the  water4  where  they 
are  attracted  to  the  eggs  by  substances 
secreted  by  its  protoplasm.  The  higher 
sexual  condition,  as  in  (Edogonium, 
where  the  gametes  are  eggs  and  sperms,  is  called  heterogamy 
because  the  gametes  are  dissimilar,  in  contrast  to  isogamy  (see 
account  of  Ulothrix,  Sec.  217). 

221.  The  Chaetophoraceae.*  The  members  of  this  family, 
including  such  common  genera  as  Stigeodonium,  Draparnaldia 

*  To  THE  INSTRUCTOR  :  It  does  not  seem  to  be  generally  known  that  forms 
of  the  Chcetophoracece  are  almost  equally  good  types  for  the  study  of  zoo- 
spores  as  Ulothrix  and  may  be  readily  substituted  for  that  form.  Stigeodonium 


of  the  Chattophoracece 


zoospores  formed  singly  in 
the  cells 


COLEOCHJETE 


191 


(Fig.  183),  and  Clicetoplwra,  are  inore 
complex  than  Ulotlirix,  for  they  consist 
of  branching  filaments  of  peculiar  forms. 
However,  the  cell  structure,  life  his- 
tories, methods  of  reproduction,  and  low 
sexual  conditions  (isogamy)  of  these 
types  all  show  relationships  to  the  Ulo- 
thricacece.  They  are  of  especial  interest 
as  leading  up  from  the  level  of  Ulo- 
thrix  to  the  highest  form  of  the  Con- 
fervales,  the  genus  Coleochcete. 

222.  Coleochaete.  Coleochcete  (family 
Coleochoetacece)  contains  a  number  of 
species  which  live  in  fresh  water,  at- 
tached to  the  stems  and  leaves  of  water 
weeds,  and  they  frequently  appear  on 
the  sides  of  aquaria.  Some  of  the  forms 
are  circular  flat  plates  or  cushions  of 
cells  that  really  consist  of  systems  of 
filaments  radiating  out  from  a  center.  ElG- 184-  Cladophora 
Large,  two-ciliate  zoospores  are  formed  This  large,  much-branched,  fii- 

A  amentous  alga,  A,  has  many 

singly  in  the  cells.  The  female  organ, 
oogonium,  is  a  large,  flask-shaped  cell 
with  a  long  neck  (Fig.  185  A,  o).  Its 
protoplasm  forms  a  single  spherical  egg 
which  nearly  fills  the  lower  swollen 
portion  of  the  oogonium,  and  the  neck 
finally  opens  above  to  allow  the  en- 
trance of  the  sperms.  The  male  organs, 
antheridia  (Fig.  185,  A,  a),  are  small 
cells,  generally  in  groups,  each  of  which  develops  a  two-ciliate 
sperm. 

and  Draparnaldia  are  common  on  stones  in  clear  brooks  and  in  springs. 
Cladophora  (Fig.  184)  is  also  a  good  type  for  laboratory  study  and  very 
common. 


species  common  in  both  fresh 
and  salt  water.  Zoospores, 
B,  are  formed  generally 
in  terminal  sporangia,  and 
there  are  gametes  which  fuse 
in  pairs,  C,  as  in  Ulothrix. 
The  older  cells  contain  large 
numbers  of  nuclei,  and  this 
form,  with  certain  relatives 
(family  Cladophoracese) ,  oc- 
cupy a  position  somewhat 
intermediate  between  the 
Confervales  and  Siphonales 


192 


THE  ALG^E 


The  fertilized  egg  becomes  an  oospore  within  the  oogonium 
and  is  further  protected  by  a  cellular  envelope  (Fig.  185,  £>) 
developed  from  short  filaments  which  grow  up  around  the 
structure  (Fig.  185,  A,f),  making  a  conspicuous  fructification. 

The  oospore  germinates  the 
following  spring,  forming 
within  itself  a  small  group 
of  cells  (Fig.  185,  C),  each  of 
which  gives  rise  to  a  zoo- 
spore.  The  decay  and  ruptur- 
ing of  the  fructification  allows 
the  zoospores  to  escape  and 
start  new  Coleochcete  plants 
at  the  beginning  of  each  sea- 
son. The  fructification  of  Co- 
leochcete serves  to  multiply 
FIG.  18o.  Coleochcete  pulmnata 

the  number  of  zoospores  pro 

A,  filaments  with  an  oogonium  o,  anthend- 

ium  a,  and  a  sperm  above ;  /,  fertilized  duced  by  the  OOSpore,  whicll 

egg  in  its  oogonium  becoming  surrounded  ig  d     rl     Rn  advantage, 
by  short  filaments  from  the  cell  below ;  J 

JB,  oospore  completely  inclosed  in  a  eel-          Coleochcete    stands    at    the 

™;;±t t^™™sr*  ^  <*  °°*  <*  ^  ***^ 

oospore,  each  cell  in  the  interior  develops    fined  lines  of  evolution  ill  the 
a  zoospore. — After  Pringsheim 


ft  ^  which   ^^    afc 

the  lowly  level  of  the  Ulothricacece  and  runs  upwards  through 
the  Chcetophoraccce.  Authors  have  generally  regarded  these 
forms  as  leading  almost  directly  to  the  bryophytes,  with  Coleo- 
chcete just  a  little  below  the  liverworts;  but,  as  we  shall  see 
later,  there  are  serious  difficulties  in  the  way  of  this  view.1 
Nevertheless,  these  forms  are  perhaps  nearer  than  any  other 
living  algae  to  the  theoretical  main  line  of  ascent. 

1  The  fructification  of  Coleochcete  has  been  regarded  as  similar  to  the  so- 
called  fruit  or  sporophyte  of  the  liverworts,  but,  as  explained  in  Sec.  336, 
there  is  strong  evidence  against  this  interpretation.  The  one-celled  sexual 
organs  of  Coleochoete  are  also  very  different  from  the  many-celled  sexual 
organs  of  the  bryophytes,  and  this  is  also  evidence  against  the  existence  of  a 
close  relationship  between  the  groups  (see  Sec.  299). 


THE  POND  SCUMS  AND  DESMIDS 


193 


ORDER  3.  THE  POND  SCUMS  AND  DESMIDS,  OR  CONJUGALES 

223.  The  pond  scums  and  desmids.  The  pond  scums  and 
desmids  (order  Conjugates)  are  remarkable  for  the  beauty  and 
symmetry  of  their  cell  structure,  and  especially  for  their  large 
and  complicated  chrornatophores.  There  are  no  motile  stages  in 
their  life  histories,  and  the  sexual  processes  consist  in  the  union 
or  conjugation  of  similar  nonmotile  gametes  (isogamy).  These 
characters  distinguish  the  group  sharply  from  all  other  algse,  but 


FIG.  186.  Desmids 

A,  Closterium,  a  vegetative  cell  at  the  left  and  a  zygospore  at  the  right  between 
the  halves  of  two  empty  cells  whose  contents  have  fused ;  B,  Cosmarium;  the 
desmid  at  the  right  has  just  divided  and  is  forming  two  new  parts  between  the 
old  halves  of  the  parent  cell.  C,  Micrasterias,  a  very  elaborate  form  in  its  out- 
line and  markings ;  D,  Hyalotheca,  a  common  filamentous  desmid ;  the  appear- 
ance of  the  cells  in  face  view  is  shown  at  the  right 

make  the  relationships  of  the  forms  very  uncertain.  These  plants 
live  only  in  fresh  water  and  seek  the  sunshine,  being  abundant 
in  clear,  shallow  pools.  The  desmids  live  chiefly  along  the  mar- 
gin and  on  the  bottom,  while  the  pond  scums  frequently  form 
growths  upon  the  surface  of  the  water,  which  appear  frothy 
because  of  the  bubbles  of  gas  (largely  oxygen)  held  among  the 
filaments.  The  filaments  are  very  slippery  to  the  touch  on 
account  of  a  slimy  excretion  from  the  cells. 

224.  The  desmids.  There  are  about  one  thousand  species  in 
this  large  group  whose  forms  are  generally  one-celled,  although 
some  desmids  are  filamentous  (Fig.  186,  D).  Each  cell  has  two 


194 


THE 


parts,  which  duplicate  one  another  even  in  the  details  of  pro- 
toplasmic structure,  and  the  nucleus  lies  in  the  middle  region, 
with  the  chromatophores  arranged  symmetrically  in  the  halves. 

The  desmids  multiply  rapidly  by 
cell  division,  each  daughter  cell 
taking  one  half  of  the  old  cell 
wall  and  adding  to  it  a  repro- 
duction of  the  other  half  (Fig. 
186,  B).  The  gametes  are  naked 
protoplasts,  which  escape  by  the 
breaking  apart  of  the  halves 
of  the  cells  and  fuse  in  pairs, 
forming  thick-walled  zygospores 
(Fig.  186,  A).  In  some  common 
forms  (Closterium  and  Cosma- 
rium)  the  zygospore  on  germi- 
nating produces  two  desmids. 

225.  The  pond  scums.  Some 
of  the  commonest  and  most 
beautiful  of  filamentous  green 
algae,  such  as  Spirogyra  and  Zyg- 
nema,  belong  here.  The  com- 
plex chromatophores  with  their 

A,  Spirogyra,  illustrating  stages  in  the    Qi1firr.-iv  rlifTprpntintprl  iwrpnnirk 

conjugation  between  cells  of  differ-  sharply  ditt  erentiated  pyrenoids 
ent  filaments,  two  zygospores  shown  give  an  especial  interest  to  the 

above;  since  the  cells  in  the  filament         -,-,  JIT       ±      j-  4.-         •  i 

on  the  left  are  shorter  than  those  on    cells    and    helP    to    distinguish 

the  right,  some  of  them  must  be  left  the  genera.    Thus  the  chroma- 

out  in  the  pairing  of  the  gametes.    ,      -,  £   &    •  i 

^,  the  conjugation  between  adjacent  tophores  of  Spirogyra  are  spiral 

cells  of  the  same  filament  in  Spirogyra    bands  (FigS.  168,187);  Zygnema 
quadrata:  C.Ziiqneina(Ziiqoqoniurt%)    ,  111 

pectinatum,  showing  zygospores  has  two  star-shaped  chromato- 

formed  in  the  conjugating  tubes  be-    phores,    and     Mougeotid     has    a 
tween  two  filaments  1,1-      i        i    •       ,1 

broad,  thin  band  in  the  center 

of  the  cell.  The  method  of  sexual  reproduction  is  very  charac- 
teristic, but  exceptional  among  the  algae.  Generally  the  cells  of 
different  filaments  unite  or  conjugate  with  one  another  by  the 


FIG.  187.   Pond  scums 


THE  DIATOMS  195 

fusion  of  processes,  one  put  out  from  each  gamete  (Fig.  187,  A). 
The  gamete  protoplast  of  one  of  the  cells  in  the  pair  then 
passes  over  into  the  other,  or  in  certain  forms  the  two  protoplasts 
unite  more  or  less  midway  between  the  two  cells  (Fig.  187,  C). 
In  some  species,  however,  there  is  a  conjugation  between  adja- 
cent cells  of  the  same  filament  (Fig.  187,  B),  the  contents  of 
one  cell  entering  the  other.  The  fusion  of  the  two  gamete  pro- 
toplasts results  in  a  zygospore,  which  develops  a  heavy  wall 
about  itself  as  in  the  desmids  and,  as  a  resting  spore,  carries 
the  plant  over  unfavorable  seasons.  On  germination  each  zygo- 
spore puts  forth  a  filament,  which  grows  by  repeated  cell  divisions 
all  along  its  length. 

ORDER  4.  THE  DIATOMS,  OR  DIATOMALES 

226.  The  diatoms.  The  diatoms  (order  Diatomales)  comprise 
a  remarkable  group  of  one-celled  plants,  containing  several  thou- 
sand species,  everywhere  present  in  both  fresh  and  salt  water. 
They  compose  the  greater  part  of  the  floating  microscopic  life, 
called  the  plankton,  of  the  ocean  and  lakes,  and  are  the  most 
important  source  of  food  for  some  of  the  smaller  animal  forms, 
and  through  them  for  the  fish  life.  The  cell  walls  are  filled  with 
the  mineral  silica  and  consist  of  two  parts,  called  valves,  which 
fit  together  something  like  the  halves  of  a  pill  box  (Fig.  188,  A). 
Diatom  cells  have  a  great  variety  of  forms,  elliptical  and  cir- 
cular, wedge-shaped  and  triangular,  cylindrical  and  rhomboidal 
(see  Fig.  188).  The  cells  are  solitary  in  many  forms,  but  in 
others  are  arranged  in  filaments,  or  borne  at  the  ends  of  gelati- 
nous stalks,  or  held  in  filamentous  sheaths  or  jelly-like  masses. 
Many  of  the  diatoms,  and  especially  the  boat-shaped  forms, 
glide  to  and  fro  in  the  water.  The  cells  contain  chromato- 
phores  which  are  generally  colored  brown  (although  certain 
species  are  green),  but  in  spite  of  this  color  the  most  natural 
position  of  the  diatoms  seems  to  be  among  the  green  algae,  with 
possible  distant  relationships  to  the  desmids. 


196 


THE  ALG^E 


The  diatoms  resemble  the  desmids  in  the  similar  halves  of 
the  cell  and  in  the  development  of  a  peculiar  type  of  spore  called 
an  auxospore,  which  probably  corresponds  to  a  zygospore.  Some 

auxospores  are  formed  by 
the  fusion  of  two  gamete 
protoplasts  which  leave 
the  diatom  shells  at  one 
side  (Fig.  188,  B),  and 
these  are  true  zygospores 
very  similar  to  those  of 
the  desmids  (Fig.  186,  A). 
Other  auxospores  are  de- 
veloped without  proto- 
plasmic fusions  and  are 
probably  examples  of  sex- 
ual degeneration ;  that  is, 
cells  which  were  origi- 
nally gametes  now  develop 
directly  into  auxospores. 
The  auxospores  are  rest- 
ing spores  and  appear  to 
be  formed  after  long 
periods  of  vegetative  cell 
divisions  to  stimulate  or 


FIG.  188.   Diatoms 

A,  Navicula,  the  boat  diatom,  the  side  view  at 
the  right  showing  the  two  overlapping  shells 


or  valves;  J3,  Acnanthes,  an  auxospore  with     rejuvenate  the  protoplasm 

for  further  activities. 
The  shells  of  diatoms, 


the  four  empty  shells  of  the  two  diatoms 
whose  contents  united  to  produce  this  sex- 
ually formed  spore  similar  to  the  zygospore 
of  the  desmids  (see  Closterium  (Fig.  186,  A) ; 


C,  Tabellaria,  groups  of  cells   united  with     being  composed  of  silica, 


one  another  to  form  a  zigzag  filament;  D, 
Licmophora,  groups  of  cells  borne  on  gelati- 


resist  solution   in   water 


nous  stalks ;   ~E,  Epithemia ;  F,  Triceratium.     and  are  constantly  being 

-B,  after  West  deposited  at  the  bottom 

of  seas,  lakes,  ponds,  and  marshes,  sometimes  in  such  quantities 
as  to  form  so-called  siliceous  or  diatomaceous  earths.  There  are 
some  geological  deposits  (Tertiary)  of  diatomaceous  earth  many 
feet  in  thickness,  as  that  at  Eichmond,  Virginia.  Some  of  these 


THE  SIPHON  ALG.E  197 

earths  have  so  large  a  proportion  of  hard  diatom  shells  as  to 
he  valuable  as  polishing  powders,  and  they  are  also  used  as  the 
absorbent  of  nitroglycerin  in  the  manufacture  of  dynamite. 

ORDER  5.   THE  SIPHON  ALG^E,  OR  SIPHONALES 

227.  The  siphon  algae.    The  siphon  algae  (order  SipJionales) 
differ  from  all  other  groups  of  algse  in  the  striking  peculiarity 
that  the  protoplasm,  with  thousands  of  nuclei,  is  not  separated 
into  compartments  or  cells,  but  is  all  contained  within  a  com- 
mon filament  or  other  cell  cavity.    Such  a  many-nucleate  struc- 
ture is  called  a  ccenocyte  (meaning  a  vessel  in  common).    The 
siphon  alg«  are  chiefly  marine,  and  many  large  and  complicated 
forms  are  found  in  the  warmer  seas  (Caulerpa,  Udotea,  etc.). 
Some  of  these  are  heavily  incrusted  with  lime  (Acetabularia, 
Penicillus,  Halimeda,  etc.).    Two-ciliate  zobspores  and  gametes 
are  developed  by  certain  types  in  cells  cut  off  from  the  ends  of 
the  filaments.    The  gametes  fuse  in  pairs  on  their  escape  into 
the  water,  forming  zygospores.    All  of  the  siphon  algse  are  isoga- 
mous,  when  sexual  at  all,  except  the  green  felt,  Vaucheria,  which 
stands  quite  alone  as  the  only  heterogamous  type  in  the  group. 

228.  Vaucheria.    Vaucheria,  the  green  felt,  is  a  very  common 
alga,  forming  mats  of  coarse  filaments  on  the  muddy  bottom  -of 
shallow  pools  and  ditches.    Some  species  are  terrestrial  and  may 
often  be  found  as  thread-like  growths  over  the  damp  earth  of 
flowerpots  in  greenhouses.    The  filaments  are  long  and  sparingly 
branched,  and  are,  of  course,  continuous  tubes    without    cross 
wralls  except  where  reproductive  organs  are  developed.    Immense 
numbers  of  small,  disk-shaped  chloroplasts   (Fig.  189,  F)  are 
present  in  the  layer  of  protoplasm  under  the  cell  wall,  and  the 
very  small  nuclei  lie  among  them.    The  protoplasm  contains 
numerous  globules  of  oil,  which  in  this  plant  takes  the  place  of 
starch  as  the  first  visible  product  of  photosynthesis. 

The  zoospores  of  Vaucheria  are  very  large,  many-nucleate 
and  many-ciliate  structures,  visible  to  the  naked  eye.    They  are 


198  THE  ALG^E 

developed  singly  in  terminal  cells  (sporangia),  which  are  cut  off 
at  the  ends  of  the  filaments  by  the  formation  of  a  cross  wall 
(Fig.  189,  A).  The  protoplasm  in  the  sporangium  contains 
hundreds  of  nuclei  from  the  beginning,  and  pairs  of  cilia  are 
formed  all  over  the  surface  of  the  protoplast  opposite  them 


FIG.  189.   Asexual  reproduction  of  the  green  felt  (Vaucheria) 

A,  formation  and  discharge  of  the  large,  many-ciliate  zoospore  from  the  terminal 
sporangium ;  B,  the  zoospore  showing  the  ciliated  surface  ;  C\  section  through 
the  surface  of  the  zoospore  showing  the  pairs  of  cilia  above  the  nuclei  and 
the  layer  of  plastids  beneath ;  D,  germination  of  zoospore ;  E,  young  plant  of 
Vaucheria,  the  two  filaments  having  arisen  at  opposite  ends  of  the  zoospore, 
one  having  developed  an  organ  of  attachment  or  holdfast  h ;  F,  a  group  of 
plastids,  the  lower  in  process  of  division. — A,  B,  after  Gotz;  6',  after  Stras- 
burger;  D,  E,  after  Sachs 

(Fig.  189,  C).  The  entire  mass  of  protoplasm  then  slips  out 
from  the  end  of  the  sporangium  (Fig.  189,  B)  and  swims  slowly 
away,  but  soon  comes  to  rest  and  puts  forth  one  or  more  fila- 
ments (Fig.  189,  D).  The  nuclei  and  pairs  of  cilia  in  this  inter- 
esting zoospore  of  Vaucheria  unquestionably  represent  the  two- 
ciliate  zoospores  characteristic  of  most  of  the  Siphonales.  and 
the  green  algae  in  general.  The  protoplasmic  divisions  neces- 
sary to  cut  out  the  numerous  zoospores  in  a  sporangium  are 


VAUCHERIA 


199 


suppressed  in  Vaucheria,  so  that  the  entire  mass  of  protoplasm 
remains  together  as  a  many-nucleate  and  many-ciliate  unit, 
which  is  really  a  protoplast  or  cell  in  spite  of  its  complicated 
structure.  Some  authors  have  regarded  this  zoospore  as  a  com- 
pound structure, — that  is,  a  mass  of  small,  two-ciliate  zoospores, 


FIG.  190.    Sexual  reproduction  of  the  green  felt  (Vaucheria) 

A,  Vaucheria  sessilis ;  o,oogonium;  a,  antheridium ;  os,  the  thick-walled  oospore, 
and-beside  it  an  empty  antheridium ;  B,  Vaucheria  geminata,  a  short  lateral 
branch  developing  a  cluster  of  oogonia  and  a  later  stage  with  mature  oogonia 
o  and  empty  antheridium  a;  C,  sperms;  D,  germinating  oospore. —  C,  after 
Woronin  ;  D,  after  Sachs 

—  but  it  is  more  correct  to  consider  it  a  large,  undivided,  many- 
nucleate,  protoplast. 

The  sexual  organs  of  Vaucheria  are  oogonia  and  antheridia, 
sometimes  found  side  by  side,  as  in  Vaucheria  sessilis  (Fig. 
190,  A),  and  sometimes  in  groups  on  special  side  branches,  as  in 
Vaucheria  geminata  (Fig.  190,  .Z?).  The  oogonium.  is  a  large 
oval  cell  separated  from  the  parent  filament  by  a  wall,  and  each 
develops  a  single  egg  (Fig.  190,  A,  B,  o).  The  young  oogonium 
contains  numerous  nuclei,  but  all  of  these  degenerate  except 
one,  which  lies  near  the  center  of  the  cell  and  becomes  the 
single  nucleus  of  the  egg.  The  antheridium  is  a  cell  formed 


200 


THE  ALG^E 


FIG.  191.   Botrydium  and 
Protosiphon 

These  forms  of  the  siphon  algae 
are  almost  indistinguisha- 
ble in  the  vegetative  condi- 
tion. The  plants  are  little 
green  globes,  somewhat 
larger  than  pin  heads,  at- 
tached to  the  surface  of  mud 
and  wet  earth  by  a  branch- 
ing system  of  filaments 
(rhizoids).  These  s-ingle 
coenocy  tic  cells  are  therefore 
differentiated  into  a  green 
part  above  ground,  exposed 
to  the  air  and  sunlight,  and 
a  colorless  underground  por- 
tion in  contact  with  mois- 
ture. A  single  cell  may  thus 
show  the  same  relation  of 
parts  as  a  complex  plant  with 
aerial  stems  and  leaves  and 
a  subterranean  root  system. 


at  the  end  of  a  branch,  which  is  gener- 
ally bent  like  a  crook  (Fig.  190,  A,  B,  a), 
and  discharges  a  very  large  number  of 
small,  two-ciliate  sperms  (Fig.  190,  C). 
The  sperms  enter  the  oogonium  through 
a  pore  in  the  beak-like  extension  at  one 
side,  and  one  of  them,  fusing  with  the 
egg,  fertilizes  it.  The  fertilized  egg  sur- 
rounds itself  at  once  with  a  heavy  wall, 
becoming  an  oospore  (Fig.  190,  A,  os), 
which  is  a  resting  spore  in  this  form,  as 
in  Volvox,  CEdogonium,  Coleochcete,  etc. 
Vauclieria  has  been  made  the  sub- 
ject of  some  interesting  experimental 
studies  on  the  conditions  which  deter- 
mine the  formation  of  zoospores  and 
sexual  organs  respectively.    Zoospores 
are  generally  developed  at  once,  follow- 
ing some  marked  change  in  the  exter- 
nal conditions,  as  in  the  character  of 
the  water,  or  an  increase  in  light  expo- 
sure, or  a  rise  in  temperature.     Sexual 
organs  are  formed  when  plants  are  cul- 
tivated in  a  2-5  per  cent  solution  of 
cane  sugar  at  a  fairly  high  temperature 
(50°-68°  F. ;   10°-20°  C.),  and  in  the 
presence  cf    light.    The  conditions  in 
Vaucheria  probably  illustrate  very  well 
the  kinds  of  influences  which  cause  an 
alga  to  begin  its  various  forms  of  fruc- 
tifications, but  very  few  algse  have  been 
studied  in  detail. 

229.  The  coenocyte.  The  large,  many- 
nucleate  structures  called  ccenocytes, 
so  well  illustrated  by  the  siphon  algae 


THE  STONEWORTS  201 

and  such  fungi  as  the  molds  (Sec.  261)  and  water  molds 
(Sec.  262),  are  peculiar  to  plants.  The  question  may  be  asked, 
Why  are  coenocytes  considered  single  cells,  and  not  a  com- 
pound structure  made  up  of  a  united  mass  of  protoplasts  repre- 
sented by  the  numerous  nuclei  ?  It  is  known  that  the  nuclei 
do  not  occupy  fixed  positions  in  the  coenocytes,  as  if  they 
represented  the  cells  of  a  compound  structure.  On  the  con- 
trary, they  shift  with  the  movements  of  the  protoplasm  which 
behaves  as  a  unit,  like  a  gigantic  protoplast  growing  in  differ- 
ent directions  in  obedience  to  various  stimuli,  and  carrying  on 
the  usual  cell  activities.  For  these  reasons  the  co3nocyte  must 
be  regarded  as  a  many-nucleate  cell  and  not  a  compound  struc- 
ture or  mass  of  protoplasts. 

ORDER  6.   THE  STONEWORTS,  OR  CHARALES 

230.  The  stoneworts.*  The  stoneworts  (order  Charales)  are 
the  most  complex  of  the  green  algse.  The  plant  body  (Fig.  192,  A) 
consists  of  long,  jointed  stems,  which  bear  circles  of  lateral 
branches  at  the  joints.  The  sexual  organs  (Fig.  192,  B,  C,D)  are 
borne  on  these  branches,  but  are  too  complicated  for  considera- 
tion here.  Many  of  the  Charales  are  heavily  incrusted  with 
lime,  which  peculiarity  gives  them  their  popular  name  of  stone- 
worts.  They  sometimes  grow  in  great  masses  attached  to  the 
bottom  of  ponds  and  shallow  lakes. 

Some  forms  of  stoneworts  (Nitella),  which  are  free  from 
incrusting  lime,  frequently  illustrate  very  beautifully  the  move- 
ments of  protoplasm  in  the  large  cells  (internodes)  which  lie 

*  To  THE  INSTRUCTOR  :  The  Charales  is  such  a  highly  special  group  that 
it  is  hardly  wise  to  give  it  much  attention  in  a  general  course,  especially  if 
time  and  material  is  available  for  a  more  thorough  study  of  the  Confervales. 
Nevertheless,  material  of  the  stoneworts  is  frequently  easily  obtained,  espe- 
cially in  the  Middle  West,  where  it  is  difficult  to  do  justice  to  the  brown  and 
red  algse,  and  it  might  be  substituted  for  certain  work  in  those  groups.  One 
of  the  best  accounts  of  the  Charales  will  be  found  in  Goebel,  Outlines  of 
Classification  and  Special  Morphology  of  Plants. 


202 


THE  ALGvE 


between  the  circles  of  lateral 
branches.  The  protoplasm 
passes  in  two  streams  in  op- 
posite directions  somewhat 
diagonally  around  the  cell. 
The  edges  of  the  stream 
form  a  line  of  stationary 
protoplasm  (free  from  chlo- 
roplasts),  because  the  move- 
ments of  the  currents,  which 
may  be  seen  on  either  side 
of  the  line,  neutralize  one 
another. 

The   Charales   stand   en- 
tirely by  themselves  at  the 
end  of  a  line  of  ascent  whose 
developmental  history  is  a 
mystery.    They   are   very 
far  above  most  of  the  green 
algse  in  the  complexity  of 
the  plant  body  and   sex- 
ual organs,  which  are  not, 
however,  like  those  of  the 
liverworts  and  mosses.   The 
FIG.  192.  The  stonewort  (Cham)          antheridium   of    the   stone- 
A,  portion  of  plant  showing  circles  of  lat-     W0rts    is    a    V61T    puzzling 
eral  branches  at  the  joints  (nodes) ;  B,  a     structure  but  the  OOgonium 
lateral  branch  bearing  the  sexual  organs ;  --i  j  i       mi 

C,  the  sexual  organs!  o,  oogonium,  with  1S  easil7  understood.  The 
spirally  wound  filaments  encircling  the  jointed  stems  with  circles 
egg  and  forming  a  crown  above;  a,  the  P  ,  ,  ,  ,  ,  f 

antheridium,  composed  of  eight  flattened  of  lateral  brandies  are,  of 
cells  (shields),  inclosing  the  antheridial  course,  much  more  COm- 
filaments;  I),  portion  of  an  antheridial  fila-  ,.  -,  ,,  ,-.  •, 

ment,  each  cell  developing  a  single  sperm;     Plated     than     the     typical 

E,  two-ciiiate  sperms  tliallus,  but  the  simple  life 

history,  with  no  trace  of  an  alternation  of  generations,  makes  it 
necessary  to  include  them  among  the  thallophytes. 


B 


SUMMARY  OF  THE   GREEN  ALG^E  203 

SUMMARY  OF  THE  GREEN  ALG^E 

23 1\  Summary  of  the  green  algae.  The  green  algaa  comprise 
a  number  of  well-defined  groups  which  are  evidently  widely 
separated  from  one  another.  The  most  conspicuous  of  these  are 
the  Volvocacece,  the  desmids  and  pond  scums,  the  diatoms,  the 
siphon  algse,  and  the  stone  worts.  They  constitute  independent 
evolutionary  lines  of  varying  importance,  but  each  one  clearly 
developing  in  ways  peculiar  to  itself  and  quite  apart  from  the 
theoretical  main  line  of  ascent  to  the  liverworts  and  mosses 
(Bryophyta).  A  discussion  of  the  origin  of  these  groups  and 
their  possible  relationships  to  one  another  would  be  much  too 
complicated  for  the  present  account.  The  forms  of  the  green 
algaa  which  seem  to  be  nearest  to  the  main  line  of  ascent  are  in 
certain  related  families  ( Ulothricacece,  Clicetoplioracece,  and  Coleo- 
clicetacece),  but  it  is  very  doubtful  if  any  of  them  are  directly 
on  the  main  line,  and  there  are  no  living  algse  known  which 
show  clearly  the  origin  of  the  bryophytes. 

Almost  all  of  the  green  algaa  at  some  stage  in  their  life 
history  form  zoospores  or  motile  gametes  (the  sperm  being 
motile  in  heterogamous  forms).  These  ciliate  cells  point  clearly 
to  an  ancestry  of  the  green  algaa  from  groups  comprising  one- 
celled  motile  organisms  something  like  the  flagellates  (Sec.  204) 
and  lower  forms  of  the  Volvocacece  (Sec.  215).  The  formation 
of  the  zoospore  and  the  motile  gamete  is  considered  to  be  a 
return  in  the  life  history,  for  a  short  time,  to  the  primitive 
one-celled  conditions  from  which  the  various  lines  of  the  green 
algaa  are  believed  to  have  arisen.  The  motile  conditions  which 
occupy  practically  all  of  the  life  history  of  the  flagellates  and 
Volvocacece  become  reduced  to  a  short  reproductive  period  in 
most  of  the  green  algae.  The  most  important  forward  steps  in 
the  evolution  of  the  green  algaa  came  with  the  introduction  of 
long  vegetative  periods  in  the  life  histories  when  the  proto- 
plasts remained  quiet  and  formed  many-celled  plant  bodies 
(coenocytic  siphon  algaa  excepted)  of  various  structure.  All  the 


204  THE  ALG^E 

possibilities  of  development  into  such  complex  attached  organ- 
isms as  the  higher  spore  plants  and  seed  plants  were  determined 
by  those  changes  in  the  habits  of  algae  by  which  the  motile 
periods  in  the  life  history  became  reproductive  stages,  and  the 
quiescent  conditions  came  to  be  the  conspicuous  part  of  the  life 
history  as  the  vegetative  plant  body  was  gradually  developed. 

232.  Summary  of  the  reproductive  organs  and  processes  of 
the  green  algae.  Zoo  spores,  also  called  swarm  spores,  are  ciliate 
asexual  cells  (generally  two-  or  four-ciliate),  and  are  developed 
as  a  rule  numerously  (in  some  forms  singly,  or  in  twos,  fours,  etc.) 
in  a  mother  cell  called  a  sporangium,  or  zoosporangium. 

Gametes  are  sexual  cells.  The  simplest  forms  are  ciliate  and 
have  the  same  form  and  structure  (morphology)  as  the  zoospores, 
to  which  they  are  related.  These  in  the  process  of  sexual  evo- 
lution became  differentiated  into  eggs  and  sperms.  Gametes  are 
developed  in  cells  called  gametangia. 

Eggs  are  never  ciliated,  and  are  consequently  nonmotile. 
They  are  generally  large  cells,  with  abundant  chromatophores 
and  food  material.  Eggs  are  formed  in  cells  called  ob'gonia. 

Sperms,  frequently  called  antherozoids  by  botanists,  are  always 
ciliated  in  the  green  algae  and  are  very  actively  motile.  They 
are  smaller  than  zoospores  and  colorless,  or  almost  colorless. 
Sperms  are  developed  in  cells  called  antheridia,  or  a  group  of 
such  cells  is  frequently  termed  an  antlieridium. 

Isogamy  is  the  sexual  condition  in  which  the  gametes  are 
similar  in  form  and  structure ;  that  is,  they  have  the  same  mor- 
phology. They  may  differ  in  size.  The  sexually  formed  cell  is 
called  a  zygospore. 

Heterogamy  is  the  sexual  condition  in  which  the  gametes  are 
different  in  form  and  structure,  as  the  sperm  and  egg,  and  there- 
fore have  a  different  morphology.  They  are  always  very  unlike 
in  size,  but  this  does  not  make  heterogamy,  because  morphology 
does  not  deal  with  size  but  with  form.  The  egg  is  fertilized  by 
the  fusion  and  entrance  of  a  sperm  and  thus  becomes  a  fertilized 
egg,  or,  if  it  develops  a  protective  cell  wall,  an  oospore. 


I 


1 II 

!  II 


'?? 


THE  BROWN  ALG^E  205 

CLASS  III.   THE  BROWN  ALGLE,  OR  PH^OPHYCE^E 

233.  The  brown   algae.    The   Pliceopliycece   comprise  a  very 
large  assemblage  of  marine  algae,  or  seaweeds,  called  the  brown 
algae  because  their  chromatophores  are  colored  brown  instead  of 
green.    The  brown  pigment,  however,  performs  the  same  sort 
of  work  (photosynthesis)  as  the  chlorophyll  of  the  green  algae. 
The  brown  algae  can  generally  be  recognized  at  a  glance  by 
their  color,  but  the  group  is  really  separated  from   all  other 
classes  of  algae  by  certain  peculiarities  of  structure,  or  mor- 
phology.   The  plant  bodies  in  most  of  the  forms  are  very  much 
larger  and  more  complex  than  those  of  any  green  algae  and  fre- 
quently have  a  degree  of  differentiation  quite  above  that  of  the 
typical  thallus.    Indeed,  some  of  the  higher  brown  algae  have 
well-defined  stems  fastened  to  the  rocks  by  remarkable  hold- 
fasts, resembling  clusters  of  roots,  and  bearing  expanded  leaf- 
like  structures  of  complex  and  striking  forms.    Certain  types 
develop  swollen  bladders,  which  contain  considerable  oxygen, 
given  off  from  the  tissues,  and  serve  to  float  parts  of  the  plant 
in  the  water.    Besides  the  complexity  of  the  plant  body  the 
brown  algae  are  also  distinguished  by  peculiarities  of  the  repro- 
ductive organs  that  can  only  be  understood  through  a  study 
of  types.    Iodine  is  obtained  from  the  ash  of  certain  kelps  and 
rockweeds.    These  larger  brown  algae   are  also  gathered  from 
the  rocks  and  beaches  by  the  peasantry  of  certain  European 
countries  and  by  farmers  on  the  New  England  coast  and  spread 
over  farm  lands  to  fertilize  the  soil. 

234.  Life  habits.    The  brown  algae  are  most  luxuriant  in  the 
colder  waters  of  the  oceans,  where  they  form  extensive  growths 
along  the   coasts.     Some   of  the  larger  forms,  as   Fucus  and 
Ascophyllum,  are  known  as  rockweeds  because  they  cover  the 
rocks  between  low  and  high  tide  marks  with  heavy  fringes  of 
brown  vegetation  (Plate  IV).    Other  forms,  known  as  the  kelps, 
or  devil's  aprons,  grow  below  or  just  at  low  watermark  and  some- 
times form  large  beds  attached  to  the  rocks.    These  larger  types 


206 


THE  ALG^E 


can  withstand  the  beating  of  the  heaviest  surf  because  of  their 
firm  texture  and  strong  holdfasts,  and  some  of  them  grow  on 
the  most  exposed  points  and  reefs.  There  are,  however,  many 
smaller  brown  alga3,  membranous  and  cord-like  forms,  and  some 

delicate  filamentous 
types  (Ectocarpus) 
which  are  as  simple  as 
many  green  algae  and 
grow  generally  in 
rather  quiet  waters. 

We  can  only  illus- 
trate the  brown  algae 
by  representatives  of 
three  orders,  —  the 
Ectocarpus  group,  the 
kelps,  and  the  rock- 
weeds. 

235.  Ectocarpus. 
This  alga  (order  Ecto- 
carpales)  is  a  branching 
filamentous  type  which 
forms  tufts  attached 

to  the  larger  alg;e,  eel- 

,  J      ,.  , 

grass>  and  to  the  wood- 
work  ot  wharves.  Its 

chief  interest  for  US  lies 
m  the  reproductive  or- 


which  are  of  two 


FIG.  193.   A  filamentous  brown  alga  (Ecto- 
carpus  siliculozics) 

A,  unilocular  sporangia,  one  containing  zoospores, 
the  other  empty  ;  a  zoospore,  z,  shown  at  the  left  ; 
B,  plurilocular  sporangia,  the  larger  mature,  the 
smaller  still  showing  the  outlines  of  the  original 
cells  in  the  branch  from  which  it  arose;  C,  the 
union  of  the  gametes  to  form  the  zygospore; 
note  that  the  chromatophores  with  the  pigment  Sorts,  and  illustrate 
spots  remain  separate.  —  C,  after  Oltmanns  very  well  the  reproduc- 

tive processes  of  the  lower  brown  algse.  The  asexual  organs  are 
one-celled  sporangia  (Fig.  193,  A),  which  develop  large  numbers 
of  kidney-shaped  zoospores,  each  with  a  pair  of  cilia  attached 
at  the  side  (Fig.  193,  A,  z).  Because  the  zoospores  are  all  de- 
veloped in  a  single  cell,  the  sporangia  are  called  unilocular 


ECTOCARPUS 


207 


sporangia,  to  distinguish  them  from  the  sexual  organs,  but  the 
structure  is  clearly  the  same  as  that  of  the  one-celled  sporangium 
of  the  green  algse. 

The  sexual  organs  are  devel- 
oped from  side  branches,  most 
of  whose  cells  divide  repeatedly 
until  an  immense  number  of 
small  compartments  are  formed. 
The  filament  thus  becomes 
transformed  into  a  complicated 
many-celled  organ  (Fig.  193,  B) 
made  up  almost  wholly  of  small 
cubical  cells,  each  of  which  de- 
velops a  single  two-ciliate  ga- 
mete similar  to  a  zoospore,  or, 
perhaps,  two  or  three  of  these 
motile  elements.  Because  the 
gametes  are  developed  in  small 
compartments,  the  organ  has 
been  termed  a  plurilocular  spo- 
rangium. It  is  clear  that  this 
many-celled  organ  is  a  very  dif- 
ferent sort  of  structure  from  the 
one-celled  reproductive  organs. 
It  marks  an  important  advance 
in  the  evolution  of  reproductive 
structures  in  plants  and  suggests 
the  many-celled  sexual  organs 
characteristic  of  the  bryophytes 
and  pteridophytes. 

The  gametes  are  known  to 
fuse  in  pairs  (Fig.  193,  C),  as  in 

,  ,  i 

many  simple  green  algae,  and 
since  they  have  a  similar  struc- 
ture, the  sexually  formed  cell  is 


FIG.  194.   Kelps  from  the  North 

Atlantic 

,  the  simple  type  of  Laminaria,  some 
of  whose  species  grow  to  be  thirty  or 
more  feet  long;  B,  the  digitate  type 
(Laminaria  digitatd),  which  is  never 
very  long,  but  is  broad  at  the  base 


208 


THE  ALG^E 


a  zygospore  and  the  sexual  condition  is  that  of  isogamy.  It  is 
interesting  to  note  that  these  motile  cells  sometimes  germinate 
without  conjugation,  just  like  the  zobspores  which  they  resem- 
ble, —  a  fact  which  shows  that  sexuality  has  not  become  very 
firmly  established  in  the  simplest  of  the  brown  algae  and  illus- 
trates, as  in  Ulothrix  (Sec.  217),  the  general  conditions  which  are 
to  be  expected  with  the  origin  of  sex  in  any  group  of  plants. 

236.  The  kelps.  The  kelps  (order  Laminariales),  also  known 
as  the  devil's  aprons,  are  the  largest  types  of  the  brown  algae. 
Those  of  the  North  Atlantic  coast  have  comparatively  simple 


FIG.  195.  The  giant  kelp  (Macrocystis) 
Adapted  from  Hooker  and  Harvey 


forms  (Fig.  194).  There  is  always  a  stalk  (stipe)  attached  to  the 
rocks  by  a  holdfast  consisting  of  a  cluster  of  strong  outgrowths, 
and  the  stalk  bears  a  long,  leaf-like  expansion  called  the  Made. 
The  blades  of  some  kelps  are  divided  lengthwise  into  segments, 
as  in  Laminaria  digitata  (Fig.  194,  B). 

Certain  kelps  of  the  Pacific  coast  are  much  more  complex, 
consisting  of  numerous  large,  leaf-like  blades  variously  arranged 
on  different  forms  of  stems.  Some  of  the  stems  attain  great 
lengths.  Thus  the  giant  kelp  (Macrocystis,  Fig.  195)  has  been 
reported  six  hundred  to  nine  hundred  feet  long,  which  is  two 
or  three  times  the  height  of  the  giant  redwoods  of  California. 
The  sea  otter's  cabbage  (Nereocystis,  Fig.  196)  frequently  has  a 
stem  more  than  one  hundred  feet  long,  which  is  enlarged  above 


THE  KELPS  209 

into  a  hollow  float  that  rests  on  the  surface  of  the  water  and 
bears  a  number  of  strap-shaped  leaves.  The  sea  palm  (Postelsia, 
Fig.  197)  is  another  remarkable  form,  with  a  thick,  strong  stalk 
about  a  foot  high,  which  bears  a  crown  of  small  leaves  and 
somewhat  resembles  a  palm  tree  in  miniature.  Macrocystis  and 


FIG.  196  FIG.  197 

FIG.  196.  The  sea  otter's  cabbage  (Nereocystis) 
Adapted  from  Postels  and  Ruprecht 

FIG.  197.  The  sea  palm  (Postelsia) 

Nereocystis  grow  in  deep  water,  but  the  sea  palms  are  found  on 
the  rocks,  where  the  surf  breaks  so  heavily  that  the  tough 
elastic  stems  are  bent  over  at  right  angles  by  the  force  of 
every  wave. 

The  kelps  reproduce  by  zob'spores    developed  in  one-celled 
sporangia  that  are  formed  in  large  patches  upon  the  leaves. 


210 


THE  ALG^E 


There  is  no  method  of  sexual  reproduction  known,  and  this 
is  remarkable,  for  groups  as  highly  developed  and  eminently 
successful  as  the  kelps  almost  always  have  well-established 

and  complex  methods 
of  sexual  reproduction. 
237.  The  rock- 
weeds.  The  rock- 
weeds  (order  Fucales) 
are  the  highest  forms 
of  the  brown  alga3, 
both  in  vegetative 
structure  and  because 
of  the  complex  sexual 
conditions  (heterog- 
amy),  with  character- 
istic eggs  and  sperms. 
The  commonest 
genus  is  Fucus  (Fig. 
198),  which  is  very 
widely  distributed  in 
the  colder  seas  and 
forms  the  bulk  of  the 
algal  vegetation  be- 
tween tide  marks 
(Plate  IV).  The  plant 
body  of  Fucus  forks 
very  regularly  (dichot- 
omous  branching), 
and  the  growth  is  from 
a  region  of  cells  situ- 
ated in  a  pit  at  the  end 


EIG.  198.   A  rockweed  (Fucus  vesiculosus) 

A,  habit  sketch,  showing  the  forking  of  the  branches ; 
6,  air  bladders;  r,  swollen  fruiting  tips  (recepta- 
cles) ,  with  the  sunken  cavities  (conceptacles)  which  The  Sexual 


of  each  branch   (Fig. 

\ 
?  p). 


contain  the  sexual  organs  ;  p,  pit  at  a  growing 
point.    JB,  base  of  a  plant  ;  h,  holdfast 


arise    from   the    sides 


FIG.  199.   The  sexual  organs  of  a  rockweed  (Fucus  vesiculosus) 

A,  section  of  a  female  conceptacle  with  oogonia,  showing  the  hairs  which  pro- 
ject through  the  opening  of  the  conceptacle  into  the  water,  and  the  loose  net- 
work of  filaments  in  the  interior  of  the  plant;  B,  mature  oogon,ium  containing 
eight  eggs;  (7,  the  discharge  of  the  eggs  from  an  oogonium;  D,  a  group  of 
antheridia  on  the  hranching  filaments  which  grow  in  tufts  over  the  sides  and 
bottom  of  the  male  conceptacles ;  E,  sperms  very  highly  magnified,  showing 
elongated  form  and  the  two  cilia  at  the  side ;  F,  an  egg  lying  free  in  the  water 
and  surrounded  by  sperms.  —  B,  (7,  D,  E,  F,  after  Thuret 

211 


212 


THE  ALG^E 


and  bottom  of  small  cavities,  called  conceptacles  (Fig.  199,  A), 
which  are  developed  in  swollen  tips  of  older  branches  termed 
receptacles  (Fig.  198,  r).  Some  species  of  Fucus  (as  F.  edentatus) 
have  both  male  and  female  organs  in  the  same  conceptacle,  but 
in  other  species  they  are  formed  in  different  conceptacles,  and 
even  upon  different  plants,  as  in  F.  vesiculosus.  The  female  organ 
is  a  large  cell,  or  oogonium  (Fig.  199,  B,  C},  which  in  Fucus 
develops  eight  eggs.  The  male  organs,  antheridia,  are  also  single 
cells  (Fig.  199,  D),  but  they  are  generally  borne  in  dense  clusters 
upon  branching  stalks,  and  each  produces  more  than  a  hundred 
very  small  sperms  with  two  cilia  at  the  side 
(Fig.  199,  E).  The  eggs  and  sperms  are 
forced  out  of  the  conceptacles  by  the  swell- 
ing of  mucilage  that  is  developed  within 
the  structure,  aided  by  the  contraction  of 
the  tissue  when  the  plants  are  exposed  at 
low  tide  to  the  drying  action  of  the  air. 

The  eggs  are  fertilized  in  the  sea  water 
outside  of  the  conceptacles.  The  sperms 
gather  around  an  egg  in  great  numbers 
(Fig.  199,  F),  making  it  revolve,  and  finally 
one  enters.  The  male  nucleus  passes  rap- 
idly to  the  center  of  the  egg  and  in  a  few 

minutes    begins  to  fuse  with  the  female 
FIG.  200.   Sargassum  ,  ,  .,  .  . 

Filipendula  nucleus.    The  striking  differences  m  size 

Tipofpiantshowingieaf-    and  structure  between  the  large  egg  and 
like  lateral  branches,    minute  sperm  make  the  sexual  process  of 
hollow  fl^a'ts^and  the    Fiicus  one  of  the  best  illustrations  of  heter- 
f ruiting  branches,  or    ogamy  in  the  plant  kingdom.     Such  ferti- 
lized eggs  as  are  fortunate  enough  to  find 
favorable  resting  places  begin  to  germinate  within  twenty-four 
hours,  and  develop  directly  into  young  Fucus  plants. 

238.  Sargassum.  Sargassum  (Fig.  200)  is  one  of  the  Fucales 
that  deserves  special  mention  for  the  complexity  of  its  plant 
body,  which  bears  three  forms  of  lateral  structures:  (I)  thin, 


THE  &ED  ALG^E  213 

leaf-like  branches  which  resemble  foliage,  (2)  berry-like  floats, 
and  (3)  small  reproductive  branches,  or  receptacles  (Fig.  200,  r). 
Some  species  of  Sargassum,  when  torn  away  from  their  at- 
tachment to  rocks,  are  able  to  vegetate  in  the  open  sea,  where 
they  are  called  gulf  weed,  but  they  are  not  known  to  fruit  in 
the  free  floating  condition.  Certain  ocean  currents  carry  and 
accumulate  immense  quantities  of  this  floating  gulf  weed  in 
great  eddies  in  the  ocean,  forming  the  Sargasso  seas. 

239.  Summary  of  the  brown  algae.    The  Phceophycece  stand 
entirely  apart  from  the  green  algae  as  a  side  line  of  plant  evolu- 
tion.   There  is  much  evidence  that  it  is  a  group  of  very  ancient 
origin,  probably  arising  from  an  ancestry  of  motile  organisms 
(somewhat  like  the  flagellates)  just  as  did  the  green  algae  in 
early  geological  ages.    The  brown  algae  have  developed  in  their 
own  peculiar  ways  the  largest  and  most  complex  forms  of  plant 
bodies  in  the  thallophytes,  and  also  some  very  high  types  of 
sexual  reproduction.    It  is  clear,  however,  that  these  have  no 
relation  to  higher  plants,  bryophytes  and  pteridophytes,  and  are 
also  entirely  independent  of  other  groups  of  algae.    Thus  heter- 
ogamy  in  the  brown  algae  has  been  developed  entirely  independ- 
ently of  heterogamy  in  the   green  algae,  illustrating  very  well 
how  similar  results  may  be  worked  out  through  different  evo- 
lutionary lines  independently  of  one  another. 

CLASS  IV.    THE  RED  ALGLE,  OR  RHODOPHYCEJE 

240.  The  red   algae.*    The  Rliodopliycece  include  the  most 
beautiful  of  the  marine  algae,  for  many  of  the  forms  are  exqui- 
sitely colored  in  clear  shades  of  red,  and  are  extremely  delicate 
in  structure.    Other  forms  are  brownish  red  or  purplish,  and  cer- 
tain types  are  greenish.    The  pigment  is  held  in  chromatophores, 

*  To  THE  INSTRUCTOR  :  In  a  brief  course  where  only  one  type  can  be 
studied  in  the  laboratory,  Nemalion  or  Batrachospermum  is  preferable,  fol- 
lowed by  a  study  of  the  life  habits  of  the  group  and  demonstrations  of 
herbarium  material. 


214 


THE  ALG^E 


and  performs  the  photosynthetic  work  of  the  plant.  Although 
the  red  algae  can  generally  be  distinguished  by  their  color, 
the  fundamental  characters  of  the  group  are  based  on  the 
structure  of  the  sexual  organs  and  certain  complications  in 
the  life  histories  which  will  be  explained  in  the  accounts  of 
Nemalion  and  Polysiplionia.  The  plant  body  of  the  red  algaa 

ranges  from  filamentous  types 
of  great  delicacy  (as  Callitliam- 
nion)  to  such  coarse  forms  as 
the  Irish  moss  (Chondrus)  and 
the  dulse  (Rhodymenia).  It  is  a 
remarkable  fact  that  the  large 
types  are  really  composed  of 
complicated  systems  of  filaments 
so  closely  associated,  however, 
as  to  give  the  appearance  of  a 
cell  tissue.  Adjacent  cells  in 
the  same  filament  are  usually 
connected  by  strands  of  proto- 
plasm, a  striking  feature  of  the 

group.     Some  of  the  red  alga?, 
FIG.  201.   The  Irish  moss  (Chondrus    as  the  Irigh  mogs  and  the  dulsej 

are  eaten,  and  a  number  of  them 

About  one  half  natural  size;  the  shaded  ,  ,.  ,         1,1 

spots  are  sexually  formed  fruits,  or  are  used  as  relishes  by  the  na- 
cystocarps  tives  of  the  Hawaiian  Islands, 

China,  and  Japan.  Certain  forms  have  an  economic  value  for 
gelatin,  which  is  obtained  from  their  tissues ;  thus  agar-agar 
comes  from  the  stems  of  a  red  alga  (Gracilaria)  which  grows 
in  the  seas  of  the  Orient. 

241.  Life  habits.  The  life  habits  of  the  red  algae  are  in  strik- 
ing contrast  to  those  of  the  brown.  They  prefer  warmer  waters, 
and  the  best  displays  are  on  such  coasts  as  the  Mediterranean, 
the  islands  of  the  West  Indies,  southern  California,  and  Aus- 
tralia, They  generally  nourish  in  deeper  waters  than  the  green 
and  brown  algae,  and  form  the  greater  part  of  the  seaweed  growth 


THE   DISTRIBUTION  OF  ALGJE  ON  ROCKS 


215 


below  the  fringe  of  green  and  brown  which  is  frequently  quite 
conspicuous  on  rocks  near  low-water  mark.  Some  of  the  forms 
are  found  at  depths  of  two  hundred  feet  or  more,  the  depth  limit 
varying  with  the  clearness  of  the  water.  Most  of  the  red  algae 
seem  to  prefer  shaded  situations  among  the  rocks,  and  it  is 
probable  that  their  characteristic  color  is  associated  with  these 
9 


FIG.  202.   The  distribution  of  conspicuous  algae  on  two  rocks  of  Spindle 
Ledge,  Woods  Hole,  Massachusetts,  in  September,  1905 

The  dotted  line  is  low-water  mark,  and  the  rocks  are  completely  covered  at  high 
tide.  Blue-green  algae:  1,  Calothrix  scopulorum,  on  the  highest  part  of  the 
rock.  Green  algae :  2,  Ulva  lactuca  var.  rigida  (sea  lettuce) ;  3,  Enteromorpha 
prolifera  (sea  lettuce) ;  both  forms  grow  well  above  low-water  mark.  Brown 
algae:  4,  Fucusvesiculosus(rockweed),  in  patches,  but  not  plentiful  in  thesummer; 
5,  Chordaria  flagelliformis  (shoestrings),  heavy  growths,  well  below  low-water 
mark.  Red  algae:  (5,  Nemalion  multiftdum,  on  the  higher  parts  of  the  rock; 
7,  Ceramium  rubrum,  a  well-marked  fringe  at  low-water  mark;  8,  Polysi- 
phonia  violacea,  a  well-marked  fringe  just  below  low-water  mark  ;  9,  Chondrus 
crispus  (Irish  moss) ,  large  patches  from  one  to  three  feet  below  low-water  mark 

peculiar  subdued  light  relations,  so  different  from  those  of  other 
algse.  Some  types  are  incrusted  with  lime  and  form  the  curious 
growths  on  rocks  called  corallines. 

242.  The  distribution  of  algae  on  rocks.  Many  seaweeds  are 
only  found  in  certain  situations  upon  rocks,  where  they  grow  in 
patches  and  fringes,  and  frequently  exhibit  a  sort  of  zonation 
somewhat  similar  to  the  distribution  of  plant  life  around  the 


216  THE  ALG^E 

• 

margins  of  ponds  and  lakes.  Fig.  202  presents  a  diagram  of 
such  distribution  on  two  isolated  rocks.  It  will  be  seen  that 
there  is  a  clear  zone  of  a  red  alga  (Polysiphonia)  just  below 
low-water  mark,  another  zone  (Ceramium)  'at  or  a  little  above 
this  mark,  while  the  Irish  moss  grows  at  some  depth.  The 
sea  lettuces  and  rockweed  are  well  above  low-water  mark,  as 
is  also  Nemalion,  which  is  exceptional  in  its  habits  for  a  red 
alga.  On  the  northerly  New  England  coast  and  beyond  there 
are  usually  two  distinct  zones  on  the  rocks,  —  one  well  above 
low-water  mark,  composed  chiefly  of  rockweeds,  and  the  other 
near  this  point,  but  below,  and  made  up  mostly  of  Irish  moss 
with  other  red  algae,  including  the  dulse. 

243.  Nemalion.  Nemalion  illustrates  excellently  the  structure 
of  the  sexual  organs  and  the  sexually  formed  fructification  of 
the  red  algee,  called  the  cystocarp  (meaning  a  fruit  cavity).  The 
plant  body  is  a  rather  soft,  cord-like,  branching  structure,  com- 
posed of  an  immense  number  of  filaments  held  together  by  a 
stiff  gelatinous  substance  around  the  cells.  There  is  a  central 
axis  of  delicate  threads,'  while  the  outer  regions  consist  of  short 
filaments  pointing  outward.  The  cells  of  the  outer  filaments 
contain  each  a  single  chromatophore,  and  the  vegetative  activi- 
ties (photosynthesis)  as  well  as  the  reproductive  take  place  in 
this  region  of  the  plant. 

The  male  organs,  or  antheridia,  consist  of  clusters  of  small  cells 
at  the  surface  of  the  plant  (Fig.  203,  A],  each  of  which  develops 
a  single  sperm,  spherical  in  form  .and  without  cilia,  and  con- 
sequently nonmotile.  The  female  organ  (Fig.  203,  B)  is  devel- 
oped at  the  end  of  a  short  branch  and  consists  of  a  cell  which 
bears  a  long,  hair-like  extension  called  the  tricliogyne  (meaning 
female  hair),  which  is  the  receptive  organ  for  the  sperms.  The 
sperms  are  applied  to  the  trichogynes  largely  by  the  contact  of 
male  plants  with  the  female  as  they  are  washed  about  by  the 
movements  of  the  water.  When  a  sperm  fuses  with  the  tri- 
chogyne  its  nucleus  (male)  passes  down  into  the  swollen  base  of 
the  female  cell  and  unites  there  with  a  female  nucleus.  The 


NEMALION 


217 


trichogyne  then  withers  above  the  fertilized  female  cell.  The 
female  cell  is  called  the  carpogonium,  but  it  corresponds  ex- 
actly to  an  oogonium,  and  indeed  resembles  very  closely  the 
oogonium  of  some  species  of  Cole- 
ocliccte  (Sec.  222).  Its  peculiar 
form,  with  a  receptive  organ,  the 
trichogyne,  is  undoubtedly  asso- 
ciated with  the  nonmotile  habits 
of  the  sperms.  The  red  algae  are 
clearly  heterogamous  in  their 
methods  of  sexual  reproduction. 

The  female  cell,  or  carpogoni- 
um, after  fertilization,  gives  rise 
to  a  dense  cluster  of  short  fila- 
ments, called  fertile  filaments, 
which  all  together  form  a  globular 
fructification  called  the  cystocarp 
(Fig.  203,Z>).  The  terminal  cells 
of  the  fertile  filaments  become 
spores,  termed  carpospores,  which 
develop  new  Nemalion  plants. 
The  cystocarp  is  clearly  a  new 
type  of  fructification  in  the  algae. 
It  is  a  structure  which  begins 
with  the  fertilization  of  the  carpo- 
gonium and  ends  with  the  forma- 
tion of  carpospores,  and  thus 
stands  as  a  phase  in  the  life  his- 
tory inserted  between  two  gener- 
ations of  the  sexual  plants. 

244.  Batrachospermum. 
Batrachospcrmum  is  one  of  the 
few  fresh-water  forms  of  the  red 
algaB,  and  is  also  an  exceptional 
type  for  its  color,  which  is 


FIG.  203.   Nemalion  multifidum 

J[,antheridia,  consisting  of  groups  of 
small  cells,  each  of  which  develops 
a  single  sperm;  the  vegetative 
branch  at  the  right  illustrates  the 
method  of  terminal  growth  and  the 
protoplasmic  connections  between 
the  cells.  B,  the  female  cell,  or  car- 
pogonium, c,  with  its  trichogyne, 
t,  to  which  are  attached  three 
sperms.  C,  early  stage  in  the  de- 
velopment of  the  cystocarp;  the 
trichogyne  above  has  begun  to 
wither.  D,  mature  cystocarp  com- 
posed of  fertile  filaments  which 
develop  the  carpospores  cs  termi- 
nally ;  wt,  withered  trichogyne 


218  THE  ALG^E 

generally  some  shade  of  green.  The  sexual  organs  and  cystocarps 
are  enough  like  those  of  Nemalion  to  make  it  as  good  a  form  to 
illustrate  the  sexual  processes  and  life  history  of  the  red  algae 
as  the  latter  type,  and  it  is  sometimes  more  available  for  inland 
classes.  Batrachospermum  grows  in  clear  brooks  and  is  generally 
found  in  its  best  condition  in  late  winter  and  early  spring. 

245.  Polysiphonia.  Polysiphonia  illustrates  some  further 
complexities  in  the  life  history  of  the  red  algse  which  are  not 
present  in  Nemalion  and  Batracliospermum.  The  filaments  of 
these  beautiful  plants  consist  of  rows  of  cells,  called  siphons, 
connected  with  one  another  in  an  elaborate  manner.  There  is 
a  central  siphon,  around  which  are  arranged  a  circle  of  outer 
siphons  variable  in  number  for  different  species. 

The  sexual  organs  are  found  011  separate  plants.  The  male 
organs,  antheridia,  are  modified  branches  (Fig.  204,  A)  that  de- 
velop an  outer  covering  of  small  cells  which  form  the  sperms 
(Fig.  204,  B).  The  female  organ  is  found  on  a  small  branch 
(Fig.  204,  C)  and  consists  of  a  carpogonium,  with  its  trichogyne, 
accompanied  by  a  number  of  vegetative  cells  which  later  take 
part  in  the  development  of  the  cystocarp.  The  fusion  of  a  sperm 
with  the  trichogyne  fertilizes  the  carpogonium  as  in  Nemalion. 
There  are  two  sets  of  activities  concerned  with  the  development 
of  the  cystocarp:  (1)  there  are  some  remarkable  cell  unions 
between  the  fertilized  carpogonium  and  neighboring  cells  (aux- 
iliary cells)  for  nutritive  purposes,  and  then  the  development  of 
carpospores  from  the  large  fusion  cell  which  is  formed ;  (2)  ac- 
companying this  activity  there  is  the  development  of  an  urn- 
shaped  envelope  (Fig.  204,  D),  from  some  of  the  vegetative  cells 
around  the  carpogonium,  and  this  is  clearly  a  protective  structure 
to  contain  the  carpospores.  The  first  set  of  activities  corresponds 
to  the  development  of  the  simple  cystocarp  of  Nemalion.  The 
second  set  forms  the  additional  urn-shaped  protective  case.  The 
cystocarp  of  Polysiphonia  is  therefore  a  system  of  two  tissues, 
one  derived  from  the  fertilized  carpogonium,  and  the  other  from 
the  vegetative  cells  of  the  parent  plant. 


POLYSIPHONIA 


219 


Besides  the  sexual  plants  (male  and  female)  there  is  an  asexual 
condition  in  Polysiplionia  called  the  tetrasporic  plant.  Tetra- 
sporic  plants  are  individuals  which  develop  asexual  spores,  called 
tetraspores  because  they  are  formed  in  groups  of  four,  termed 
tetrads,  in  mother  cells  (Fig.  204,  E,  F}.  The  tetraspore  mother 
cells  arise  from  the  central  siphon  near  the  ends  of  the  branches. 


FIG.  204.   Polysiplionia  violacea 

A,  tip  of  filament  showing  two  antheridia,  a;  B,  cross  section  of  a  portion  of  an 
antheridium  illustrating  the  development  of  the  sperms  at  the  ends  of  the  very 
numerous  short  branches ;  C,  a  procarp  with  the  projecting  trichogyne  t,  from 
the  female  cell  (carpogonium),  which  is  hidden  by  the  surrounding  sterile  cells; 
D,  mature  cystocarp  with  the  urn-shaped  envelope  inclosing  the  cluster  of 
carpospores,  a  single  spore  shown  at  the  right;  E,  a  short  branch  from  a  tetra- 
sporic plant ;  F,  two  groups  of  tetraspores  from  a  branch  similar  to  E ;  note 
the  peculiar  arrangement  of  the  tetraspores  in  a  group  of  four,  or  tetrad 

Some  recent  investigations  clearly  indicate  that  the  tetrasporic 
plants  come  from  carpospores,  and  that  the  tetraspores  develop 
into  sexual  plants.  So  there  is  an  alternation  of  sexual  and 
tetrasporic  plants  in  the  life  history  of  Polysiplionia. 

246.  Summary  of  the  red  algae.    It  is  quite  certain  that  the 
red  algae  have  had  their  origin  from  a  very  much  higher  level 


220  THE 

than  the  green  or  brown  algae  because  of  the  complicated  sexual 
organs  and  life  histories  and  the  absence  of  motile  stages  repre- 
sented by  the  zoospores  and  motile  gametes  of  those  groups. 
The  red  algae  resemble  ColeocJicete  (Sec.  222)  in  a  number  of 
features,  and  it  is  possible  that  this  type  may  be  rather  close  to 
the  starting  point  of  the  group.  The  peculiar  structure  of  the 
female  sexual  organ  (carpogonium),  which  is  really  anoogonium 
with  a  receptive  organ  (trichogyne),  is  undoubtedly  associated 
with  the  loss  of  motility  on  the  part  of  the  sperms.  But  the 
most  remarkable  peculiarity  is  the  development  from  the  ferti- 
lized carpogonium  of  a  tissue  which  produces  carpospores.  This 
structure  (from  the  fertilized  carpogonium  to  the  carpospores) 
is  a  new  phase  in  the  life  history  of  algae,  and  together  with 
protective  envelopes,  when  present,  constitutes  the  cystocarp.  It 
is  probable  that  the  asexual  tetrasporic  plants  found  in  most 
species  of  the  red  algae  arise  from  the  carpospores,  and  that  the 
sexual  plants  in  these  types  are  developed  from  the  tetraspores. 
The  structure  developing  from  the  fertilized  carpogonium  and 
ending  with  the  carpospore,  together  with  the  tetrasporic  plant, 
when  present,  therefore  forms  an  asexual  phase  in  the  life  history, 
alternating  with  the  sexual  plants.  Such  asexual  phases  are 
called  sporophytes  (meaning  spore-bearing  plants),  to  distinguish 
them  from  the  sexual  plants,  called  gametophytes  (meaning 
gamete-bearing  plants),  and  their  following  one  after  another 
in  a  life  history  constitutes  an  alternation  of  generations.  Such 
an  alternation  of  generations  is  found  in  very  few  groups  of 
the  thallophytes,  but  it  is  characteristic  of  the  life  histories 
of  all  higher  groups  beginning  with  the  liverworts  and  mosses 
(Sec.  285).  Its  significance  is  discussed  in  Chapter  xxvi. 


CHAPTEK  XXI 

SUMMARY   OF  THE  LIFE  HISTORIES  AND  EVOLUTION  OF 
THE  ALGJE 

247.  The  life  histories  of  the  algae.  The  life  history  of  a 
plant  is  the  succession  of  stages  leading  from  one  generation  to 
another  and  of  course  includes  the  reproductive  periods.  Kepro- 
duction  may  be  as  simple  a  process  as  the  breaking  off  of 
portions  from  the  parent  plant,  called  vegetative  reproduction. 
Almost  all  groups  of  plants  have  developed  some  forms  of 
vegetative  reproduction.  The  detached  portions  may  be  merely 
fragments,  as  in  Oscillatoria  (Sec.  209),  or  much  more  compli- 
cated, as  certain  bud-like  structures  in  some  of  the  brown  and 
red  algae.  Methods  of  vegetative  reproduction  give  very  simple 
life  histories,  which  are  merely  a  succession  of  similar  forms 
such  as  may  be  represented  by  the  formula 

P-P-P-P,  etc., 
P  standing  for  the  plant  type. 

The  commonest  methods  of  reproduction  in  the  algae  are 
through  the  special  cells  called  spores,  which  may  be  asexual 
in  character  or  formed  sexually.  The  commonest  form  of  spore 
reproduction  in  the  algae  is  through  the  zoospore.  When  there 
is  no  sexual  process  in  the  life  history,  but  some  method  of 
asexual  spore  reproduction,  the  formula  of  the  life  history 
becomes  p  _  asex^  s  _  P  _  asex^  s  _  P  etc  ? 


asex.  s.  standing  for  asexual  spore. 

The  development  of  sex  in  a  plant  complicates  at  once  the 
life  history.  Gametes  are  formed,  which  unite  to  give  sexually 
formed  cells  or  spores.  These  spores  may  develop  directly  into 
plants  like  the  parents,  as  in  Splicerella  and  Volvox,  Spirogyra, 

221 


222    LIFE  HISTORIES  AND  EVOLUTION  OF  THE  ALG^E 

Vaucheria,  Chara,  and  Fucus,  or  they  may  form  zoospores,  which 
swim  off  and  grow  at  once  into  plants  like  the  parents,  as  in 
CEdogonium,  Ulothrix,  and  Coleochcete,  the  latter  type  developing 
the  zoospores  somewhat  indirectly  through  a  group  of  cells.  But 
in  all  these  forms  the  essentials  of  the  life  history  are  expressed 
by  the  formula 


^>sex.  s.  —  P^^>sex.  s.  —  P,  etc., 
^  ^9 

g  and  sex.  s.  standing  for  gamete  and  sexually  formed  spore, 
respectively. 

It  must  always  be  remembered  that  the  algae  with  sexual 
methods  of  reproduction  frequently  have  also  asexual  zoospores 
or  other  forms  of  asexual  reproduction,  which  may  produce  a 
number  of  successive  generations  between  the  development  of 
sexual  plants.  This  happens  very  frequently  among  the  green 
algae  (as  in  Ulothrix,  CEdogonium,  Coleochcete,  and  Vaucheria) 
and  in  the  lower  brown  algae  (as  Ectocarpus).  The  life  history 
formula  of  the  sexual  algae  may  then  be  variously  broken  by  the 
introduction  of  successive  generations  developed  asexually. 

In  the  red  algae  the  sexually  formed  cell  corresponding  to 
an  oospore  does  not  give  rise  at  once  to  a  generation  like  the 
parent  plant,  but  an  asexual  generation  is  inserted  between 
successive  sexual  plants,  alternating  with  them.  This  asexual 
phase  may  be  represented  by  the  tissue  which  produces  the  car- 
pospores  within  the  cystocarp,  or  it  may  be  represented  by  this 
structure  plus  the  tetrasporic  plant.  There  is,  then,  in  the  red 
algae  an  alternation  of  generations,  an  asexual  phase,  or  sporo- 
phyte  (the  cystocarp  and  tetrasporic  plant),  alternating  with  the 
sexual  plant,  or  gametopliyte.  This  is  the  most  complex  type  of 
life  history  in  the  algae  and  may  be  expressed  by  the  formula 

/cystocarp  and      \  /carpospore  andx 

G  <C  Q  >  —  Si  tetrasporic  plant,  1  —  asex.  s.(  tetraspore, 

\when  present       /  \when  present 

>  —  S  —  asex.  s.  —  G,  etc., 
9 


THE  EVOLUTION   OF   SEX 


223 


G  and  S  standing  for  the  gametophyte  and  sporophyte  gener- 
ations, respectively.  It  will  appear  later  that  the  higher  plants 
have  essentially  the  same  life  history  formula  as  this. 

248.  The  evolution  of  sex.  The  account  of  the  algae  has 
given  material  for  a  brief  discussion  of  the  origin  and  evolution 
of  sex.  It  has  been  shown  that  the  simplest  forms  of  sexual 
cells,  or  gametes,  have  essentially  the  same  structure  and  origin 
as  the  zoospores.  These  conditions  are  illustrated  by  Sphcerella, 
Ulotlirix,  Ulva,  and  Cladophora.  The  difference  between  the 
gamete  and  zoospore  is  chiefly  one  of  size.  The  gametes  are 
smaller  because  they  are  generally  formed  more  numerously  in 
their  mother  cells.  Gametes  sometimes  are  able  to  germinate 
like  zoospores,  but  such  gametes  are 
apt  to  develop  small  and  weak  plants, 
and  as  a  rule  they  must  fuse  with 
one  another  in  pairs  in  order  to  live. 
It  seems  clear  that  sex  arose  with 
the  development  of  a  type  of  zoospore 
smaller  and  apparently  weaker  in  its 
power  of  vegetative  growth  than  the 
normal  zoospore.  These  smaller  zoo- 
spores  formed  the  habit  of  fusing  in 
pairs,  and  this  habit,  finally  becoming 
fixed  in  the  plant's  life  history,  de- 
veloped into  a  method  of  sexual  repro- 
duction. 

After  the  establishment  of  sex  in  a    FlG-  205-  Cutleria  mulffida 
group  of  plants,  further  developments  A>  the  larse  female  gamete; 

.  />,  the  same  at  rest  and  sur- 

will  tend  to  modify  the  form  of  the 

gametes,  the  process  finally  ending  in 

their    differentiation    into    eggs    and 

sperms.    The  simplest  gametes  are  so  similar  in  form  and  size 

that  they  cannot  be   distinguished  as   male  and  female,  but 

a  number  of  algse  have  gametes  which  are  different  in  size, 

although  similar  in  structure,  or  morphology.    This  condition 


rounded  by  small  male  ga- 
metes, one  of  which,  tn,  is 
shown  in  the  act  of  fusion 


224     LIFE  HISTORIES  AND  EVOLUTION  OF  THE  ALG^E 

has  been  noted  in  species  of  Chlamydomonas  (Fig.  177),  and  some 
other  green  algse  (as  the  siphon  alga  Bryopsis)  show  it,  while 
among  the  brown  algse  certain  species  of  Ectocarpus  and  Cut- 
leria  (Fig.  205)  furnish  especially  good  illustrations.  The  larger 
gamete  is  female  and  often  has  a  relatively  short  motile  period, 
being  fertilized  when  at  rest  by  the  smaller  male  gamete,  thus 
resembling  an  egg  (Fig.  205,  7?).  These  are  transitional  condi- 
tions leading  towards  the  highest  types  of  gametes,  —  the  egg 
and  sperm.  The  term  isogamy  (similar  gametes)  is  applied  to 
sexual  conditions  when  the  gametes  are  similar  in  form, — 
that  is,  have  the  same  structure,  or  morphology,  even  though 
they  may  be  very  different  in  size.  The  sexually  formed  cell  is 
called  a  zygofepore. 

Heterogamy  (dissimilar  gametes)  is  the  sexual  condition  in 
which  the  gametes  are  unlike  inform,  —  that  is,  have  a  dif- 
ferent structure,  or  morphology,  one  being  the  larger  nonnio- 
tile  egg,  and  the  other  the  small,  specialized,  motile  sperm.  The 
sexually  formed  cell  is  called  an  oospore,  or  the  egg  is  said  to 
be  fertilized  after  the  union  with  the  sperm.  Several  eggs  may 
be  formed  in  the  mother  cell,  or  oogonium,  as  in  Fucus,  but 
they  are  generally  developed  singly.  This  latter  condition  is  the 
result  of  evolutionary  processes  by  which  all  of  the  protoplasm 
and  nutritive  material  in  the  oogonium  is  preserved  for  a  single 
egg,  thus  giving  it  all  the  energy  and  power  of  growth  possible. 
The  sperms,  on  the  contrary,  are  frequently  developed  in  very 
great  numbers  in  their  parent  cells,  and  are  consequently  small, 
and  must  die  quickly  if  they  are  unable  to  fertilize  eggs. 

It  is  very  important  to  note  that  the  principles  affecting  the 
evolution  of  sex  are  always  at  work  and  have  undoubtedly 
operated  separately  in  various  groups  of  plants.  Thus  heter- 
ogamy  has  developed  independently  in  the  lines  of  the  green 
algse,  ending  in  Volvox,  CEdogonium,  Coleochcete,  Vaucheria,  and 
Char  a,  and  in  the  rockweeds  (Fucales)  as  well.  Heterogamy 
is  the  highest  point  of  sexual  evolution,  but  plants  above  the 
thallophytes  show  some  important  advances  in  their  methods 


THE  EVOLUTION  OF   THE   ALG^  225 

of  protecting  the  egg.  It  will  appear  later  that  the  eggs  of  liver- 
worts, mosses,  and  ferns  are  retained  in  special,  protective,  cellular 
structures  called  arcliegonia,  which  are  the  female  reproduc- 
tive organs.  The  presence  of  this  organ  is  one  of  the  important 
characters  of  these  groups  (bryophytes  and  pteridophytes),  and 
its  absence  is  one  of  the  peculiarities  of  the  thallophytes. 

249.  The  evolution  of  the  algae.  The  evolution  of  the  algae 
is  the  result  of  many  factors  which  affect  their  life  habits  and 
life  histories.  Sexual  processes  have  been  the  chief  factors 
modifying  life  histories,  for  they  are  always  a  stimulus  to  devel- 
opment, and  have  been  the  starting  points  for  some  of  the  most 
important  complications  in  the  life  histories  and  developments 
of  groups.  The  most  conspicuous  illustration  of  this  principle 
appears  in  the  red  algae,  where  an  asexual  generation  follows  the 
sexual  process,  and  similar  conditions  are  present  in  a  peculiar 
group  of  the  brown  algae  represented  by  Dictyota. 

One  of  the  most  clearly  marked  evolutionary  principles  illus- 
trated in  the  algae  is  the  tendency  to  establish  fixed  or  attached 
forms  and  to  limit  the  motile  stages  in  the  life  history  to  the  re- 
productive cells  (zoospores  or  gametes).  Some  of  the  lower  algae 
are  motile  throughout  almost  all  of  their  life  histories,  as  in  the 
Volvocacece  (Sec.  215)  and  that  group  of  uncertain  relationships, 
the  flagellates  (Sec.  204).  But  the  motile  stages  become  merely 
reproductive  phases  in  the  higher  forms.  Thus  the  appearance 
of  zoospores  and  motile  gametes  in  the  life  histories  of  higher 
types  of  algae  is  believed  to  represent  a  return  for  a  short  time 
to  the  motile  conditions  and  habits  of  their  ancestors. 

The  establishment  of  attached  plant  bodies  opened  immense 
possibilities  of  plant  development,  and  resulted  at  once  in  a 
great  variety  of  structures.  The  first  of  these  were  simple  forms 
of  thalli,  such  as  filaments,  membranes,  and  plates  of  cells.  But 
later  the  plant  structures  became  more  complex,  developing 
holdfasts  and  stems,  which  bore  leaf-like  lateral  structures, 
evidently  differentiated  to  give  a  large  exposure  to  sunlight, 
and  for  the  work  of  photosynthesis.  Thus  types  of  plant  bodies 


226     LIFE  HISTORIES  AND  EVOLUTION  OF  THE 

arose  which  were  more  complex  than  the  thallus,  since  they 
showed  three  regions,  —  stems,  leaf-like  blades,  and  holdfasts. 
We  shall  see  later  that  the  stems,  blades,  and  holdfasts  of  the 
highest  algae  do  not  correspond  to  the  stems,  leaves,  and  roots  of 
fern  and  seed  plant,  which  developed  very  much  later  through 
a  complicated  history.  But  this  differentiation  of  the  plant  body 
in  the  thallophytes  is,  at  least,  a  response  to  the  same  sort 
of  influences  as  guided  the  development  of  the  higher  plants. 
These  influences  were  in  part  the  evident  advantages  to  a  plant 
of  being  fastened  to  a  suitable  attachment,  from  which  it  can 
grow  and  present  as  much  surface  as  possible  to  the  sunlight. 

In  conclusion,  one  should  think  of  the  algae  as  comprising  a 
large  number  of  divergent  lines,  whose  relationships  are  some- 
times so  distant  that  one  cannot  make  even  a  good  guess  as  to 
the  evolutionary  history.  The  stoneworts  (Char ales}  constitute 
perhaps  the  best  illustration  of  such  an  isolated  group.  Very 
few  of  the  algae  now  living  are  near  the  theoretical  main  line 
of  ascent  to  the  liverworts  and  mosses.  The  algae  should  be 
thought  of  as  spreading  out  in  many  directions,  each  group 
developing  in  its  own  particular  line  of  evolution,  adjusting 
itself  as  best  it  may  to  its  particular  sort  of  life.  Some  pos- 
sible relationships  have  been  suggested  in  the  accounts  of  the 
various  groups,  but  the  subject  is  too  complex  to  be  given 
detailed  consideration  here. 


CHAPTER   XXII 

THE  FUNGI  AND  THEIR  RELATION  TO  FERMENTATION 
AND  DISEASE 

250.  The  fungi.*  The  fungi  are  thallophytes  whose  plant 
bodies  have  no  chlorophyll  or  other  coloring  matter  capable 
of  doing  the  work  of  photosynthesis.  Consequently  fungi  are 
unable  to  manufacture  the  primary  foods  of  plants,  such  as 
starch,  and  are  absolutely  dependent  upon  organic  substances 
obtained  from  animals  and  plants.  Fungi  must  therefore  live 
either  as  parasites  upon  living  plants  or  animals,  called  their 
hosts,  or  as  saprophytes  (meaning  decay  plants)  upon  dead  organic 
matter  or  the  products  of  decay.  Fungi  are  frequently  spoken  of 
as  colorless  plants,  because  they  have  no  chlorophyll,  but  many 
forms  are  brilliantly  colored  by  special  pigments. 

The  fungi  have  undoubtedly  been  derived  from  the  algae,— 
not  from  a  single  group  of  the  algae,  however,  but  from  several 
widely  separated  groups.  Consequently  the  classes  of  the  fungi 
have  not  developed  one  from  another,  but  in  most  cases  are 
believed  to  be  either  of  entirely  independent  origin  or  of  very 
remote  relationship  through  ancient  forms  of  algae  no  longer 
living.  The  chief  peculiarities  of  the  structures  and  life  histo- 
ries of  fungi  are  largely  the  results  of  their  adaptations  to  lives 
of  parasitism  or  saprophytism.  One  of  the  results  of  these  adap- 
tations has  been  the  development  of  a  much  greater  number  of 
species  than  is  found  in  the  algae. 

*  To  THE  INSTRUCTOR  :  As  in  the  account  of  the  algae,  this  chapter  describes 
more  forms  than  should  be  given  in  a  general  course.  Many  of  them  must 
be  omitted  or  merely  discussed  in  the  class.  They  have  been  included  in  order 
to  provide  a  range  of  material  for  selection  adaptable  to  various  sections  of 
the  country  and  the  different  conditions  under  which  the  subject  must  be 
presented. 

227 


228  THE  FUNGI 

We  shall  consider  five  classes  in  the  series  of  the  fungi  among 
the  thallophytes  (see  Outline  of  Classification,  p.  155). 

Class  V.       The  bacteria,  or  Schizomycetes. 
Class  VI.     The  yeasts,  or  Saccharomycetes. 
Class  VII.    The  alga-like  fungi,  or  Phycomycetes. 
Class  VIII.  The  sac  fungi,  or  Ascomycetes. 
Class  IX.     The  basidia  fungi,  or  Basidioinycetes. 


CLASS  V.  THE  BACTERIA,  OR  SCHIZOMYCETES 

251.  The  bacteria.  The  bacteria  are  the  smallest  living 
beings  known.  The  single  cells  of  many  species  are  less  than 
one  ten  thousandth  of  an  inch  in  diameter,  and  some  are  very 
much  smaller  still.  Most  of  the  bacteria  are  one-celled.  Some 
types  are  spherical  or  oval,  some  are  straight  or. slightly  bent 
rods,  and  some  are  spirally  twisted  forms  of  various  lengths 
(Fig.  206).  Certain  species  are  provided  with  numerous  cilia  and 
are  actively  motile.  The  cells  may  be  loosely  joined  together 
in  chains  or  collected  in  jelly-like  masses  or  colonies,  which  are 
sometimes  brightly  colored,  yellow,  red,  blue,  or  green.  Some  of 
the  bacteria  are  filamentous  and  made  up  of  rows  of  cells.  The 
cells  are  very  simple  in  structure,  since  they  do  not  have  a 
clearly  defined  nucleus,  and  in  this  important  respect  they 
resemble  the  blue-green  algce,  from  which  they  are  believed  to 
have  been  derived  (Sec.  211). 

The  cells  of  the  bacteria  multiply  by  simply  splitting  apart, 
which  gives  them  their  name  of  Schizomycetes,  or  fission  fungi. 
These  cell  divisions,  under  favorable  conditions,  take  place  in 
some  forms  as  frequently  as  once  every  half  hour,  and  the  de- 
scendants from  a  single  individual  may  number  many  millions 
in  a  few  days.  The  bacteria  are  only  limited  in  their  remarkable 
powers  of  multiplication  by  lack  of  food  or  other  unfavorable 
conditions.  Many  bacteria  have  the  power  of  developing  thick- 
walled  resting  cells,  or  spores,  within  the  parent  cell,  which  can 
survive  a  temperature  above  the  boiling  point  of  water  and  also 


D 


FIG.  206.    Groups  of  bacteria  stained  to  show  their  cilia 

Bacillus  subtilis,  an  organism  of  decay  characteristic  of  hay  infusions ;  B,  Bacil- 
lus typhi,  the  germ  of  typhoid  fever;  C,  Bacillus  vulgaris,  single  cells  and 
filaments ;  D,  Planococcus  citreus,  which  forms  yellow  colonies  on  various  sub- 
strata ;  E,  Pseudomonas  syncyanea,  which  turns  milk  blue ;  F,  other  species 
of  Pseudomonas ;  G,  Microspira  comma,  the  germ  of  Asiatic  cholera ;  H,  Spi- 
rittum  undula,  in  water  containing  decaying  fish,  alga?,  etc. ;  /,  another  species 
of  Spirillum.  — After  Migula 

229 


230  THE  FUNGI 

below  freezing,  and  are  able  to  live  for  very  long  periods.  But 
other  forms,  as  the  Bacillus  of  typhoid  fever  (Fig.  206,  B),  may 
be  certainly  killed  within  a  few  minutes  by  boiling  the  water 
in  which  they  live.  Certain  bacteria,  as  the  species  which  pro- 
duce lockjaw  and  cause  butter  to  become  rancid,  will  live  with- 
out air,  and  are  even  injured  by  contact  with  free  oxygen.  They 
obtain  the  oxygen  necessary  for  respiration  from  compounds, 
such  as  the  carbohydrates,  which  contain  it. 

The  bacteria  are  present  almost  everywiiere,  floating  in  the 
air  on  particles  of  dust,  in  the  water,  in  the  soil,  and  always 
living  within  and  upon  the  bodies  of  animals.  Thus  the  bacte- 
ria are  ready  to  grow  and  multiply  wherever  they  find  favorable 
conditions,  but  these  are  exceedingly  various  for  the  different 
species.  Some  forms  are  restricted  to  a  parasitic  life  on  particu- 
lar hosts,  as  certain  animals  or  plants,  or  man.  Other  types  are 
connected  with  special  chemical  reactions,  as  in  the  processes  of 
decay,  fermentation,  nitrification,  etc.  Many  bacteria  are  indis- 
pensable to  life  on  the  earth,  and  of  the  greatest  service  to  man. 
Many  forms  are  harmless,  but  of  no  special  value  to  man. 
Some  cause  dangerous  contagious  diseases. 

252.  Decay.  Decay  is  the  destruction  or  decomposition  of 
highly  complex  organic  compounds,  such  as  the  proteids,  fats, 
sugars,  and  cell  walls  of  plants,  by  which  they  are  broken  down 
into  successively  simpler  substances,  and  finally  into  fluids  and 
gases,  some  of  which  are  very  ill  smelling.  The  products  of 
decomposition  form  various  chemical  combinations,  and  are 
finally  used  again  in  the  constructive  processes  of  life.  The  bac- 
teria and  other  fungi  are  the  chief  agents  of  decay,  and  if  it 
were  not  for  them  the  world  would  soon  be  filled  with  organic 
waste  products,  together  with  the  dead  bodies  of  animals  and 
plants  of  no  value  as  food.  Thus  all  the  chemical  elements 
capable  of  sustaining  life  would  long  ago  have  been  used  up  and 
life  on  the  earth  would  have  ceased.  The  bacteria  are  there- 
fore chiefly  responsible  for  a  circulation  of  elements  (see  dia- 
gram, Fig.  207),  from  the  highly  complex  organic  compounds  of 


DECAY  231 

animals  and  plants  back  to  the  simpler  substances  from  which 
green  plants  manufacture  their  food  and  build  up  protoplasm. 

Food  may  be  kept  indefinitely  when  under  conditions  that 
hinder  the  growth  of  bacteria,  as  in  cold  storage.  The  exclusion 
of  all  bacteria  from  hermetically  sealed  tinned  foods,  in  which 
all  germs  have  been  previously  killed  by  heat,  is  the  chief  prin- 
ciple in  the  success  of  the  canning  industry.  The  agreeable 
flavors  of  high-grade  butter  and  certain  cheeses,  as  well  as  the 
gamy  taste  of  meat,  are  largely  due  to  bacteria,  and  really  indi- 
cate the  first  stages  in  the  process  of  decay,  although  usually  not 
at  all  harmful  or  distasteful.  Not  infrequently,  however,  incip- 
ient putrefaction  forms  certain  organic  poisons,  called  ptomaines, 
in  nitrogenous  foods,  and  these  may  give  rise  to  distressing 
symptoms,  or  even  prove  fatal  to  the  consumer. 

253.  Fermentation.  Decay  rnay  take  place  in  two  very  dif- 
ferent classes  of  substances :  (1)  the  carbohydrates,  such  as  cel- 
lulose, starch,  sugar,  etc.,  and  (2)  the  proteids  or  nitrogenous 
substances  that  make  up  protoplasm,  flesh,  and  many  food  prod- 
ucts. The  breaking  down  of  the  carbohydrates  is  called  fermen- 
tation, and  many  other  fungi  besides  the  bacteria  are  concerned 
with  the  process.  Yeasts,  for  example,  are  the  most  important 
organisms  in  the  fermentation  of  sugar,  and  the  decay  of  cell 
walls  in  timber  is  chiefly  due  to  some  of  the  higher  fungi. 

The  best-known  types  of  fermentation  are  the  alcoholic  and 
the  acid.  Alcoholic  fermentation  involves  the  change  of  sugars 
to  alcohols,  accompanied  by  the  formation  of  large  quantities  of 
carbon  dioxide,  and  will  be  considered  more  especially  in  the 
account  of  the  yeasts.  Acid  fermentation  is  the  transformation 
of  sugars  and  alcohols  into  organic  acids,  and  bacteria  play  the 
most  important  part  in  this  process.  Thus  the  change  of  cider 
to  vinegar  is  one  of  sugars  and  alcohols  into  acetic  acid,  and 
the  souring  of  milk  is  the  formation  of  lactic  acid  from  milk 
sugar.  Both  processes  are  caused  by  bacteria.  There  are  a 
number  of  stages  in  the  processes  of  fermentation.  For  exam- 
ple, cellulose  is  first  changed  into  some  kind  of  sugar,  and  this 


232 


THE  FUNGI 


later  into  alcohols  and  organic  acids.  The  last  stages  result  in 
the  formation  of  the  gas  carbon  dioxide  (C02)  and  sometimes 
marsh  gas  (CH4),  which,  when  mixed  with  hydrogen  phosphide, 
becomes  the  "  will-o'-the-wisp  "  of  swamps. 


FIG.  207.   Diagram  illustrating  the  circulation  of  nitrogen 

Nitrogen  is  taken  by  green  plants  from  the  nitrates,  and  through  energy  derived 
from  the  sunlight  the  proteids  are  formed.  Animals  carry  the  process  of 
proteid  manufacture  somewhat  farther.  The  nitrogen  of  the  proteids  is  then 
returned,  through  the  decay  of  waste  products  (urea,  etc.)  and  dead  tissues,  to 
simpler  substances,  and  finally  to  ammonia,  which  is  worked  over  into  nitrates 
by  the  nitrifying  bacteria.  Free  nitrogen  is  brought  into  the  circle  by  the 
nitrogen-fixing  symbiotic  bacteria 

Some  important  forms  of  fermentation  have  no  connection 
with  living  organisms,  but  are  due  to  special  substances  called 
unorganized  ferments,  or  enzymes  (Sec.  10).  Such  a  ferment  is 
diastase,  which  converts  starch  to  sugar. 


THE  CIRCULATION   OF  NITROGEN  233 

254.  Nitrification.    The  decay  of  proteid    matter   involves, 
first,  the  change  of  the  insoluble  proteids  into  soluble  substances 
called  peptones,  —  a  similar  process  to  that  of  digestion  in  the 
stomach.    This  liquefaction  is  due  to  the   secretion  of  special 
ferments  by  certain  bacteria.    Then  follow  further  complicated 
changes  until  the  nitrogenous  substances  are  broken  down,  and 
ammonia  (NH3), a  relatively  simple  compouud,is  formed,  together 
with  various  organic  acids  and  other  compounds.    Two  forms  of 
bacteria  which  are  abundant  in  almost  all  soils  cooperate  to  trans- 
form the  ammonia  first  into  nitrous  acid,  and  then  into  nitric 
acid,  the  latter  forming  at  once  nitrates,  or  salts  of  nitric  acid. 
The  nitrates  are  the  chief  source  of  the  nitrogen  supply  of  green 
plants.    The  process  by  which  the  ammonia  of  decay  becomes 
available  through  the  nitrates  for  plant  use  instead  of  passing 
into  the  air  is  called  nitrification. 

255.  The  circulation  of  nitrogen.    There  is  a  circulation  of 
nitrogen  in  nature,  which  is  indicated  in  the  diagram  (Fig.  207). 
This  circulation  starts  with  the  nitrates,  which  are  taken  up 
in  solution  by  the  cells  of  green  plants, —  in  the  higher  plants, 
of  course,  through  the  root  system.   The  nitrogen  in  the  nitrates 
is  combined  with  carbon  compounds  obtained  from  the  carbo- 
hydrate food  manufactured  by  the  processes  of  photosynthesis. 
Hydrogen,  oxygen,  sulphur,  and  often  phosphorus  also  enter 
into  the  resultant  substances,  which  are  proteids.    The  energy 
which  makes  possible  this  building  up  of  the  complex  proteids 
comes  from  the  sunlight,  as  is  indicated  in  the  diagram.   Animals 
are  able  to  carry  the  building-up  processes  somewhat  higher, 
obtaining  their  energy  from  food  which  comes  directly  or  indi- 
rectly   from   plants.    Then  the  breaking-down  process  begins 
through  the  decay  of  nitrogenous  waste  products  and  of  dead 
matter,  and  this  is  accomplished  as  described  in  the  previous 
sections  through  the  activities  of  fungi  and  chiefly  the  bacteria. 
Finally,  simple  ammonia  is  produced,  and  this,  by  the  process  of 
nitrification,  enters  into  the  formation  of  nitrates,  and  the  nitro- 
gen is  then  available  again  for  green  plants. 


234  THE  FUNGI 

256.  The  fixation  of  free  nitrogen.  One  of  the  most  impor- 
tant relations  of  bacteria  to  agriculture  and  to  plant  life  gener- 
ally lies  in  the  ability  of  some  species  to  put  the  free  nitrogen 
of  the  air  into  chemical  compounds  that  are  available  for  absorp- 
tion by  green  plants  growing  in  barren  soil.  When  crops  are 
taken  off  the  land  through  a  series  of  years  the  supply  of  nitrates 


FIG.  208.   Tubercles  on  the  roots  of  red  clover 

I,  section  of  ascending  branches;    b,  enlarged  base  of  stem;   t,  root  tubercles 
containing  bacteria 

is  largely  used  up  and  the  soil  becomes  impoverished  or  ex- 
hausted. The  nitrogen  may  be  brought  back  to  such  soil  by 
fertilizers,  but  this  is  expensive.  The  restoration  of  nitrogen  to 
barren  land  has  been  one  of  the  most  serious  problems  of  agri- 
culture. There  is  one  of  the  bacteria  (Pseudomonas  radicicola), 
which  lives  on  the  roots  of  members  of  the  legume,  or  pea  fam- 
ily, including  such  forms  as  the  clover,  alfalfa,  and  soy  bean,  and 
develops  swollen  regions  called  root  tubercles  (Fig.  208).  This 
remarkable  organism  is  able  to  take  the  free  nitrogen  from 


THE  GERM  DISEASES  235 

the  air  and  pass  it  through  complicated  chemical  changes  to 
the  clover  and  alfalfa.  Consequently  these  crops  can  be  grown 
on  worn-out  soil  or  in  waste  land  that  is  deficient  in  nitrates. 
Indeed,  soils  may  now  be  inoculated  with  fluid  cultures  of  these 
"  nitrogen-fixing  bacteria,"  so  that  the  organisms  will  immedi- 
ately establish  root  tubercles  on  the  seedlings  of  these  legumes, 
when  sown,  or  the  seeds  themselves  may  be  soaked  in  cul- 
tures insuring  the  application  of  the  bacteria.1  Therefore,  when 
a  soil  becomes  barren  of  nitrogen  through  successive  crops  of 
wheat,  for  example,  the  nitrogen  may  be  largely  restored  by 
planting  clover  or  alfalfa  and  plowing  the  crops  under.  Barren 
soil  may  also  be  inoculated  more  certainly  by  distributing  over 
it  earth  from  an  old  clover  field. 

The  "  nitrogen-fixing  bacteria  "  make  available  the  almost  in- 
exhaustible supply  of  free  nitrogen  in  the  air  which  cannot  be 
absorbed  by  green  plants  and  which  consequently  has  been  of  no 
service  to  agriculture.  As  indicated  in  the  diagram  (Fig.  207), 
free  nitrogen  is  constantly  being  brought  into  the  nitrogen 
circle  through  the  bacteria  which  form  root  tubercles  (symbiotic 
bacteria),  and  this  helps  to  make  up  the  loss  of  nitrogen  from 
the  nitrogen  circle,  which  comes  in  various  ways,  as  by  fire  or 
the  escape  of  ammonia  into  the  air. 

257.  The  germ  diseases.  There  is  a  class  of  contagious,  and 
in  some  cases  very  dangerous,  diseases  caused  by  certain  bacte- 
ria which  are  frequently  called  microbes,  or  germs.  The  most 
serious  are  diphtheria,  typhoid  fever,  tuberculosis  (consumption), 
cholera,  leprosy,  bubonic  plague,  pneumonia,  influenza  or  grippe, 
and  whooping  cough.  Some  other  germ  diseases,  such  as  malaria, 
tropical  dysentery,  and  possibly  smallpox,  are  caused  by  lowly 
organisms  which  are  not,  however,  bacteria.  The  germ  diseases 
are  due  to  the  parasitic  development  of  the  organism  within  the 

1  See  Moore,  "  Soil  Inoculation  for  Legumes,"  United  States  Department 
of  Agriculture,  Bureau  of  Plant  Industry,  Bulletin  71,  1905,  and  Wood, 
"  Inoculation  of  Soil  with  Nitrogen-Fixing  Bacteria,1'  Bulletin  72,  Part  IV, 
1905. 


236  THE  FUNGI 

human  or  other  host.  They  are  contagious  because  the  germs 
can  be  easily  passed  directly  or  indirectly  in  various  ways  from 
the  ill  person  to  those  around  him. 

'The  active  substances  which  affect  the  patient  are  known  in 
all  cases  to  be  certain  poisons  called  toxins,  which  are,  for  the 
most  part,  secretions,  less  often  decomposition  products,  accom- 
panying the  growth  of  the  bacteria.  These  poisons  become  dis- 
tributed by  the  blood  and  cause  the  fevers.  The  body  resists 
the  effects  of  the  toxins  to  the  best  of  its  ability,  and  in  some 
cases  substances  are  formed  called  antitoxins,  which  neutralize 
the  poisons.  The  injection  into  the  human  body  of  an  antitoxin, 
which  is  obtained  from  the  blood  of  a  horse  infected  with  diph- 
theria, is  the  chief  principle  in  the  "  antitoxin  "  treatment  of  this 
very  serious  disease.  Kecovery  from  a  germ  disease  generally 
renders  the  person  safe,  or  immune  from  further  attack  for  a 
long  time,  because  the  body  has  developed  resistant  powers 
to  the  poisons  and  growth  of  that  particular  germ.  The  viru- 
lent poisons  called  ptomaines  are  usually  the  result  of  bacterial 
growths  in  foods  that  have  not  been  properly  kept. 

Inflammation  of  wounds  is  caused  by  germs,  and  the  forma- 
tion of  pus  is  in  large  part  the  gathering  of  white  blood  corpus- 
cles which  feed  on  the  germs  as  they  multiply  in  the  infected 
tissues.  The  whole  practice  of  modern  surgery  is  based  on 
absolute  cleanliness  in  the  treatment  of  wounds  to  prevent  the 
entrance  of  bacteria  during  operations. 

There  are  some  serious  bacterial  diseases  of  plants,  as  the 
pear  and  apple  blight,  cucumber  and  melon  wilt,  black  rot  of 
cabbage,  wet  rot  of  potatoes,  and  hyacinth  blight,  and  probably 
peach  yellows  is  also  of  this  class. 

258.  Public  health.  The  matter  of  public  health  and 
hygiene  calls  for  constant  attention  on  the  part  of  physicians 
and  health  officers  to  the  possible  sources  of  germ  diseases. 
For  example,  contaminated  water  and  impure  milk  are  the 
commonest  means  of  infection  for  typhoid  fever,  and  epidemics 
of  this  disease  are  frequently  traced  to  these  sources.  We 


PUBLIC   HEALTH  237 

cannot  emphasize  these  points  better  than  by  studying  the  his- 
tory of  a  typical  typhoid  epidemic,  taking  as  our  illustration 
the  well-known  outbreak  in  1885,  in  Plymouth,  Pennsylvania, 
a  town  of  about  eighty-five  hundred  inhabitants.  Typhoid  fever 
appeared  in  the  spring  with  such  violence  that  from  fifty  to 
two  hundred  cases  developed  daily,  until  about  eleven  hundred 
persons  were  stricken  (about  one  eighth  of  the  population), 
more  than  one  hundred  of  whom  died.  The  disease  appeared 
only  in  persons  who  drank  the  hydrant  water  from  certain 
town  reservoirs,  and  those  who  used  private  wells  escaped.  On 
investigation  the  following  facts  were  established.  During  the 
winter  a  case  of  typhoid  fever,  contracted  in  Philadelphia,  had 
been  cared  for  in  a  house  which  stood  close  to  a  stream  that 
flowed  into  the  town  reservoirs.  During  the  illness  intestinal 
discharges  from  the  patient  had  been  thrown  out  upon  the 
snow  within  a  few  feet  of  this  stream.  During  late  March  and 
early  April  the  snow  melted  and  there  were  frequent  rains  that 
washed  the  germ-laden  material  into  -the  stream,  which  carried 
it  into  the  reservoirs.  The  first  cases  of  typhoid  fever  in  the 
epidemic  appeared  within  two  or  three  weeks  (the  period  of 
incubation  in  typhoid  fever)  after  the  infected  water  had  been 
distributed  through  the  town.  Thus  the  entire  epidemic  was 
due  to  the  carelessness  or  ignorance  of  attendants  who  did  not 
safely  dispose  of  the  germ-filled  wastes  from  a  typhoid  patient. 
The  terrible  outbreaks  of  cholera  are  usually  due  to  infection 
of  water  supplies.  The  germs  of  tuberculosis  are  very  widely 
distributed  by  means  of  the  dried  sputum  of  diseased  persons, 
hence  the  importance  of  rules  against  spitting  in  public  places. 
The  common  diseases  incident  to  the  association  of  children  in 
school,  such  as  diphtheria,  scarlet  fever,  measles,  and  mumps, 
make  necessary  the  strict  isolation  of  all  cases  until  there  is  no 
possible  danger  of  contagion.  As  the  sources  of  germ  infection 
are  reduced  or  stamped  out,  the  possibilities  of  germ  diseases 
become  at  once  lessened.  The  healthy  human  body  is  wonder- 
fully resistant,  and  the  problem  of  public  health  is  largely  the 


238  THE  FUNGI 

practical  one  of  combating  germs.  So  important  are  the  bacte- 
ria in  disease  and  hygiene  that  a  science  has  developed,  called 
bacteriology,  with  elaborate  methods  of  its  own  to  which  special- 
ists give  their  entire  attention. 

CLASS  VI.   THE  YEASTS,  OK   SACCHAROMYCETES 

259.  The  yeasts.  The  yeasts  are  much  larger  than  the  bac- 
teria, and  have  a  more  complex  cell  structure,  for  there  is 
present  a  clearly  defined  nucleus.  The  cells  reproduce  in  a 
peculiar  manner  called  budding,  and  the  yeasts  are  frequently 
termed  budding  fungi.  Small  extensions  are  put  forth  from  the 
cells  (Fig.  209,  A),  which,  after  increas- 
ing in  size,  become  cut  off  from  the  par- 
ent structure.  The  parent  and  daughter 
cells  frequently  remain  attached  in  short 
chains  or  clusters.  The  relationships  of 
the*  yeasts  are  very  obscure,  but  there 
are  reasons  for  believing  them  to  be  de- 
generate conditions  derived  from  some 
types  of  higher  fungi  whose  spores  are 

FIG.  209.    Yeast  (Saccha-  kllOWn   at  timeS  tO   PaSS    lnt°   yeast-like 

romyces  cerevisice)  forms  when  cultivated  in  sugary  solutions. 

A,  vegetative  cells,  show-       Yeasts  are  chiefly  interesting  as   the 

ing  method  of  budding ;  agents  of  alcoholic  fermentation  by  which 

B,  spore  formation  TIT.  .        i 

sugar  dissolved  in  water  is  changed  into 

alcohol  and  the  gas  carbon  dioxide.  The  alcoholic  nature  of 
wines,  beers,  ales,  and  hard  cider  is  due  to  the  fermentation  of 
grape  juice,  wort,  or  sweet  cider,  all  of  which  contain  sugar,  and 
the  froth  and  bubbles  of  gas  which  escape  from  the  fermenting 
fluid  is  carbon  dioxide.  The  yeasts  are  distributed  very  widely, 
and  they  are  sure  to  be  introduced  by  dust  into  any  sugar  solu- 
tion that  is  not  sealed  up.  Therefore  weak  sugar  solutions  fer- 
ment spontaneously  if  left  exposed,  although  it  is  the  practice 
in  the  manufacture  of  beers  and  some  wines  to  use  special  kinds 


THE  ALGA-LIKE  FUNGI  239 

of  yeasts  that  are  cultivated  for  the  purpose.  The  yeasts  that  are 
distributed  indiscriminately  by  the  air  are  called  wild  yeasts,  to 
distinguish  them  from  those  which  are  cultivated  for  the  pur- 
poses of  brewing  and  bread  making.  The  wild  yeasts  some- 
times become  established  in  cheeses  and  other  dairy  products, 
and  also  in  breweries,  where  they  set  up  fermentations  that 
render  the  food  or  drink  unfit  for  use. 

The  raising  of  bread  results  from  the  fermentation  by  yeast 
of  sugar  that  is  present  in  the  dough.1  The  cavities,  or  holes,  in 
the  dough  are  formed  by  bubbles  of  carbon  dioxide- which,  with 
the  small  percentage  of  alcohol  developed,  is  driven  off  in  the 
baking.  Compressed  yeast  is  made  in  certain  distilleries  from 
cultures  in  large  vats,  whose  yeast  scum  is  removed  and  pressed 
into  the  yeast  cakes  that  are  sold  for  domestic  use. 

CLASS  VII.    THE   ALGA-LIKE  FUNGI,  OR 
PHYCOMYCETES 

260.  The  alga-like  fungi.    The  Pliycomycetcs  (meaning  alga- 
fungi)  comprise  a  large  number  of  forms  which  resemble  the 
alga3  in  their  structure  and  methods  of  reproduction.     Some  of 
them  are  one-celled  and  microscopic,  but  others  are  very  con- 
spicuous mold  forms,  and  certain  types  are  destructive  parasites 
that  cause  some  very  serious  plant  diseases.    The  interesting 
fungus  (Empusa)  which  kills  the  house  flies,  that  are  frequently 
found  attached  by  their  mouth  parts   to  window  panes   and 
woodwork,  is  in  a  special  group  of  this  assemblage.    We  shall 
only  be  able  to  consider  representatives  of  the  following  three 
orders  of   this  interesting  class  of   the  fungi:    (1)  the  molds, 
(2)  the  water  molds,  and  (3)  the  blights  and  rots. 

261.  The  molds.    The  molds  (order  Mucorales)  form  very  ex- 
tensive and  conspicuous  shining  cobweb-like  growths  (Fig.  210) 
through  and  upon  the  material  of  manure  heaps   and   other 

1  See  the  paper  by  Helen  W.  Atwater,  "  Bread  and  the  Principles  of  Bread 
Making,"  United  States  Department  of  Agriculture,  Farmer's  Bulletin  112, 
1900. 


240 


THE  FUNGI 


masses  of  decaying  matter.    It  is  desirable  that  the  term  mold 
should  be  restricted  to  fungi  of  this  group. 

The  bread  mold  (Rhizopus  nigricans)  illustrates  well  the 
characters  of  the  group.  An  extensive  growth  may  always  be 
obtained  on  bread  by  placing  it  in  air  saturated  with  moisture, 
as  under  a  bell  jar  set  in  a  dish  of  water.  The  vegetative  body 
consists  of  large  branched  filaments  which  generally  appear 


FIG.  210.   The  mycelium  of  a  mold  (Mucor  Mucedo)  developed  from 
a  single  spore 

a,  6,  and  c,  erect  branches  which  are  to  bear  the  sporangia,  showing  three  stages 
of  development.  —  After  Brefeld 

glistening  white  because  they  are  covered  with  minute  drops 
of  moisture.  The  individual  filament  of  a  fungus  is  called  a 
hypka  (meaning  a  web),  and  a  mass  of  hyphse  is  termed  a 
mycelium.  The  hyphse  of  the  bread  mold  resemble  the  fila- 
ments of  Vaucheria  (Sec.  228)  in  having  no  cross  partitions, 
the  filaments  being  a  single  chamber  from  end  to  end,  and 
consequently  a  coenocyte  (Sec.  229).  The  multinucleate  proto- 
plasm forms  a  layer  under  the  wall  of  the  hypha  and  contains 


THE  MOLDS 


241 


minute  globules  of  a  fatty  nature.  The  bread  mold  is  an  excel- 
lent example  of  a  saprophytic  fungus.  The  hyplise  grow  all 
through  the  porous  substance  of  the  moist  bread  and  absorb 
fluids  containing  products  of  the  bread's  incipient  decay.  The 
material  over  which  a  saprophytic  fungus  grows  and  upon 
which  it  lives  is  called  its  substratum. 

The  fructifications  of  the  bread  mold  are  very  characteristic. 
Numerous  erect  branches  arise,  several  in  a  group,  from  creeping 
hyphae  that  develop  clusters  of  short,  root-like  filaments  at  these 
points  (Fig.  211).  The  end  of  each  erect  branch  then  gradually 


FIG.  211.   Growth  habit  of  the  bread  mold  (Rhizopus  nigricans) 

Sketch  showing  two  groups  of  erect  hyphse  bearing  sporangia,  with  root-like 
clusters  of  filaments  at  their  bases 

enlarges  and  becomes  separated  from  the  stalk  below  by  a  dome- 
shaped  cross  wall  called  the  columella  (Fig.  212,  A).  The  ter- 
minal cell  becomes  a  spore  case,  or  sporangium,  and  develops  a 
multitude  of  smoke-colored  spores,  which  make  the  spore  cases 
appear  as  black  heads  upon  the  upright  stalks  (Fig.  212,  B). 
The  spores  are  distributed  by  the  breaking  of  the  sporangium 
wall,  exposing  the  dome-shaped  columella  which  remains  at  the 
end  of  the  stalks  after  the  dispersal  of  the  spores  (Fig.  212,  Z>). 
The  molds  have  a  remarkable  method  of  sexual  reproduction, 
which  is,  however,  rarely  found  in  the  bread  mold  (Rhixopus), 


242 


THE  FUNGI 


but  is  not  uncommon  in  other  genera,  as  Mucor  and  Sporo- 
dinia.  Two  short  branches  from  the  mycelium  become  applied 
to  one  another,  end  to  end  (Fig.  213,  A).  The  tip  of  each  then 
becomes  cut  off  as  a  sexual  cell,  or  gamete  (Fig.  213,  B,  C), 
peculiar  in  having  very  many  nuclei,  and  consequently  called  a 
ccenogamete.  The  two  gametes  finally  fuse,  and  a  large  zygo- 
spore  (Fig.  213,  D)  with  heavy  black  walls  is  formed  between 


FIG.  212.   The  sporangium  of  the  bread  mold  (Rhizopus  nigricans) 

A,  young  sporangium,  showing  dome-shaped  cross  wall  (columella)  shortly  after  its 
formation;  B,  mature  sporangium,  the  columella  being  hidden  by  the  spores; 
C,  diagram  of  a  lengthwise  section  of  a  sporangium  ;  s,  spore  cavity  ;  c,  col- 
umella. D,  columella  after  the  rupturing  of  the  sporangium  wall,  which  was 
attached  along  the  line  a  corresponding  to  similar  line  in  B ;  clusters  of 
spores  still  clinging  to  the  columella 

the  filaments.    It  is  probable  that  the  sexual  nuclei  from  the 
two  gametes  fuse  in  pairs  within  the  zygospore. 

262.  The  water  molds.  The  water  molds  (order  Saprolegni- 
ales)  are  very  remarkable  aquatic  fungi  which  grow  on  the  dead 
bodies  of  insects  when  immersed  in  pond  or  ditch  water  (Fig. 
214,  A).  Certain  species  attack  the  gills  and  mouths  of  young 


THE  WATER  MOLDS 


243 


fishes  in  hatcheries  and  may  be  very  destructive.  The  coenocytic 
hyphse  live  in  the  tissues  of  the  animal,  and  filaments  grow  out 
from  them  freely  into  the  water, 
where  they  develop  the  repro- 
ductive organs. 

Zoospores  are  formed  numer- 
ously in  terminal  club-shaped 
sporangia  and  are  discharged 
into  the  water  (Fig.  214,  (7,  D). 
They  are  two-ciliate  and  consti- 
tute the  method  of  rapid  mul- 
tiplication, swimming  about  in 
the  water,  seeking  a  favorable 
substratum  on  which  to  settle 
down. 

The  sexual  organs  are  male 
and  female.  Globular  oogonia 
are  formed  at  the  ends  of  cer- 
tain hyphse,  and  each  develops 
a  number  of  eggs  (Fig.  214,  F). 
The  male  organs  are  delicate 
antheridial  filaments  which 
arise  below  the  oogonia  or  from 
neighboring  hyphse.  These  ap- 
ply themselves  to  the  oogonia 
and  send  delicate  tubes  (con- 
jugation tubes)  into  the  interior, 
which  in  some  forms  are  said 
to  unite  with  the  eggs.  How- 
ever, it  is  known  that  the  antheridial  filaments  in  many  of 
the  water  molds  perform  no  function,  and  indeed  are  not  even 
present  in  some  types.  In  such  cases  the  eggs  mature  into 
oospores  without  fertilization.  The  water  molds  furnish,  then, 
excellent  illustrations  of  the  degeneration  of  a  sexual  process,  a 
phenomenon  found  in  other  groups  of  fungi. 


FIG.  213.  Formation  of  zygospores 
in  a  mold  (Mucor  Mucedo) 

A,  two  hyphaj  in  contact,  end  to  end; 
fi,  the  terminal  gametes;  C,  later 
stage,  the  gametes  fusing;  D,  a  ripe 
zygospore  ;  E,  germination  of  a  zygo- 
spore,  the  filament  forming  a  spo- 
rangium at  once  in  this  case. 
Brefeld 


After 


244 


THE  FUNGI 


The  suppression  of  a  sexual  act  is  termed  by  botanists  apog- 
amy  (meaning  without  marriage),  or  sometimes  parthenogenesis, 
when  the  egg  itself  develops  without  fertilization.  Apogamy  is 
found  in  many  groups  of  plants, —  in  the  algae  and  fungi,  among 
the  ferns,  and  even  in  the  seed  plants. 


FIG.  214.  A  water  mold  (Saproleynia  mixta) 

A,  habit  sketch  of  the  mycelium  around  a  fly ;  sporangia  being  formed  at  the  tips 
of  the  longest  hypha}  and  sexual  organs  nearer  the  body  of  the  insect;  />,  tip 
of  hypha ;  C,  terminal  sporangium  filled  with  zoospores ;  J),  empty  sporangium 
with  a  group  of  zoospores  near  the  opening;  E,  empty  sporangium  with  the 
hypha  continuing  its  growth  inside ;  f\  an  oogonium  containing  many  eggs 
and  with  three  antheridial  filaments  applied  to  it 

263.  The  blights.  The  blights  (order  Pcronosporales)  are 
parasitic  fungi  which  cause  some  very  destructive  plant  diseases. 
Some  of  them  are  also  called  "  downy  mildews,"  but  it  would  be 
better  if  the  term  mildew  were  reserved  for  a  peculiar  group  of 
sac  fungi  (Sec.  266).  The  hyphae  form  extensive  growths  in  the 
tissues  of  the  hosts.  The  asexual  fructifications  appear  on  the 
surface,  but  the  sexually  formed  oospores  are  developed  within 


THE   BLIGHTS 


245 


the  host.    The  type  most   available    for   study  is    the    blister 
blight  (Albugo),  but  the  potato  blight,  or  rot, 
and  the  grapevine  blight  (downy  mildew)  are, 
for  economic  reasons,  the  most  important  forms 
in  the  group. 

The  blister  blight.  The  blister  blight  (Albugo) 
grows  on  the  shepherd's  purse  (Capsclla)  and 
not  infrequently  on  the  radish,  appearing  as 
white  blisters  on  the  leaves  and  stems  (Fig.  215). 
The  blisters  are  formed  by  the  asexual  fructifica- 
tions, which  consist  of  masses  of  spores  called 
conidia  that  are  developed  in  chains  from  the 
ends  of  hyphse  just  underneath  the  epidermis  Blisters  containing 
(Fig.  216,  A,  B).  Conidia  are  air  spores  of  fungi,  conidia  on  the 

V      f,  '  '       .  .      .         L   .        .     .  stem  of  the  shep- 

—  that  is,  spores  formed  singly  or  in  chains  at      herd's  purse 
the  ends  of  special  branches  and  scattered  in      (Capselia) 
the  air.    Those  of  Albugo  are  distributed  by  the  wind  after  the 
breaking  of  the  blisters,  and  germinate  in  moisture,  developing 


FIG.  215 

The  blister  blight 
(Albugo  Candida) 


FIG.  216.   Reproductive  organs  of  the  blister  blight  (Albugo  Candida) 

A,  section  through  the  edge  of  a  blister  on  a  leaf;  the  air  spores,  or  conidia,  are 
f.ormed  in  chains  under  the  epidermis  from  the  swollen  tips  of  fungal  filaments 
growing  between  the  cells  of  the  leaf;  B,  tips  of  two  filaments,  showing  devel- 
opment of  the  conidia  serially ;  C,  a  filament  showing  sucker-like  structures 
(haustoria)  which  enter  the  cells  of  the  host ;  I),  the  sexual  organs ;  the  male 
cell,  or  antheridium,  a,  has  just  discharged  its  nucleus  through  a  beak-like 
process  into  the  single  egg  within  the  oogonium. 


246 


THE  FUNGI 


several  two-ciliate  zoospores.  If  the  conidium  has  germinated 
on  the  proper  host  after  a  rain  or  heavy  dew,  the  zoospores 
swim  over  the  moist  surface,  and  finally  coming  to  rest  they 


0 


put  forth  delicate  germ  tubes  that  enter 
the  host  through  one  of  the  breathing 
pores  or  stomata.  The  sexual  organs  are 
generally  found  in  portions  of  the  leaves 
and  stems  which  become  much  swollen 
and  colored  reddish  or  purplish.  The  large 
oogonium  forms  a  single  egg  and  is  ac- 
companied by  a  single  antheridial  filament 
which  develops  from  the  hypha  below 
(Fig.  216,  D).  The  antheridial  filament 
puts  forth  a  tube-like  process  which  en- 
ters the  oogonium  and  discharges  one  or 
more  nuclei  into  the  egg,  fertilizing  it. 
The  fertilized  egg  develops  heavy  walls, 
becoming  an  oospore,  which  rests  during 
the  winter,  and  on  germinating  in  the 
spring  produces  a  large  number  of  zoo- 
spores  that  infect  new  hosts. 

•The  potato  blight,  or  rot.    The  potato 

tification  of  the  potato  blight  (fkytof^thora  infcstans)  has  a  dil- 
blight  (Phytophthora  ferent  type  of  conidial  fructification  from 
infestans)  Albugo.  The  hyphse  emerge  from  the 

A,  the  air  spores  (conidia)    leaves  through  the  stoinata  (Fig.  217,  A), 

formed    on    long    fila-  .  ,.  „  i    r       i       •       .1 

merits  which  grow  out  and  conidia  are  formed  freely  in  the  air 
from  the  interior  of  the  in  immense  quantities.  These  air  spores 

potato  leaf  througll  the  ...   J    ..  .     , 

stomata;  B,  the  devel-  are  distributed  by  the  wind,  and  germi- 
opment  of  zoospores  in  nating  in  moisture  develop  zoospores  (Fig. 

a  conidium;  a  single  •  */•>••  • 

zoosporeisshownatthe     217,    B),   which    infect    116W    hosts,    as   111 

right.- After Schenck  Albugo.  Cloudy,  wet,  and  windy  seasons 
are  naturally  especially  favorable  to  the  spread  of  the  potato 
blight.  The  green  parts  of  a  blighted  potato  plant  wither,  and 
the  potatoes  either  cannot  be  formed,  or  rot  in  the  ground.  The 


FIG.  217.  Conidial  fruc- 


SUMMARY   OF  THE  ALGA-LIKE  FUNGI  247 

disease  is  carried  over  from  one  year  to  the  next  in  diseased 
potatoes  that  are  planted.  The  potato  blight  came  originally 
from  South  America  (perhaps  Peru)  and  first  appeared  in  Europe 
in  1845,  probably  introduced  from  North  America.  The  disease 
spread  very  rapidly,  causing  local  famines  in  various  countries, 
notably  in  Ireland.  It  is  now,  however,  largely  held  in  check 
by  spraying  the  plants  with  Bordeaux  mixture,  which  contains 
copper  and  is  poisonous  to  the  fungus. 

The  grapevine  blight,  or  downy  mildew.  This  genus  (Plas- 
mopara)  develops  conidia  011  hyphse  outside  of  the  host  plant, 
as  in  the  potato  blight,  but  they  germinate  by  tubes  instead  of 
forming  zoospores.  The  disease  had  its  origin  in  America,  but 
our  vines  are  not  generally  very  seriously  injured  by  it.  How- 
ever, when  it  was  accidentally  introduced  into  Europe  it  proved 
a  terrible  menace  to  the  vine-growing  industries  there.  The 
European  varieties  of  grapes  are  largely  grafted  upon  American 
rootstocks  because  the  latter  resist  the  attacks  of  a  very  destruc- 
tive insect  pest  called  Phylloxera.  But  the  American  grapevine 
blight  was  for  a  time  more  injurious  than  the  insect,  until  means 
were  discovered  of  keeping  it  in  check  by  spraying  the  .vines 
with  Bordeaux  mixture. 

The  interesting  genus  Pythium,  which  causes  the  "  damping 
off  "  of  seedlings,  and  is  sometimes  very  destructive  in  green- 
houses, is  related  to  the  blights. 

264.  Summary  of  the  alga-like  fungi.  The  chief  points  of 
resemblance  of  the  Phycomyeetes  to  certain  algse  lie  in  the 
coenocytic  structure  of  the  fungal  filaments  and  the  develop- 
ment of  zoospores  in  terminal  sporangia.  The  sexual  organs  are 
likewise  similar  to  those  of  algae  in  that  they  are  developed 
terminally,  but  there  are  important  modifications  because  motile 
sperms  are  not  generally  formed.  However,  motile  sperms  are 
known  for  one  type  (Monollepharis).  The  conidia  are  plainly 
modified  sporangia,  which  become  detached  from  the  parent  fila- 
ments and  are  distributed  as  special  reproductive  spores.  The 
algae  which  most  resemble  the  larger  filamentous  Phycomyeetes 


248 


THE  FUNGI 


are  such  forms  as  Vaucheria  (Sec.  228),  and  other  types  of 
the  Siphonales,  and  some  authors  believe  that  the  molds,  water 
molds,  and  blights  have  been  derived  from  that  general  region 
of  the  algas. 


CLASS  VIII.    THE  SAC  FUNGI,  OE  ASCOMYCETES 

265.  The  sac  fungi.  The  sac  fungi  are  distinguished  by  a 
peculiar  type  of  reproduction,  through  spores  which  are  devel- 
oped, generally  eight  in  number,  in  a  special  unicellular  organ 
called  an  ascus  (plural,  asci),  which  means 
a  sac.  The  asci  are  produced  sometimes 
in  very  great  numbers  in  a  fructification 
termed  an  ascocarp,  or  sac  fruit,  which  is 
a  structure  of  importance.  The  filaments, 
or  hyphse,  of  the  sac  fungi  are  divided  by 
cross  walls  into  cells,  and  are  never  long 
coenocytes,  as  in  the  alga-like  fungi  (Phy- 
covtiycetes).  The  Ascomycetes  is  one  of  the 
two  largest  groups  of  the  fungi,  comprising 
more  than  fifteen  thousand  species.  We 
can  only  describe  a  few  forms  from  the 
following  groups  :  (1)  the  mildews,  (2)  the 
cup  fungi,  and  (3)  the  knot  and  wart  fungi. 
FIG.  218  Sac  fruits  (as-  266  The  mildews.  The  true  mildews 

cocarps)  of   the    lilac 

mildew   (Microsphcera    (order  Perisporiales)  are  a  very  clearly  de- 

Alni)  on  the  lower  sur-    fined  group  of  fungi,  and  it  is  desirable 
face  of  a  lilac  leaf          thafc  the    term   mMew   be    restricted   to 

them.  They  are  mostly  external  parasites,  very  common  011 
the  leaves  of  many  seed  plants,  such  as  wheat,  lilac,  Virginia 
creeper,  grapes,  verbena,  cherry,  oak,  willow,  etc.  The  hyphse 
form  a  cobweb-like  growth  (mycelium)  over  the  leaves,  and  put 
forth  sucker-like  processes  called  haustoria,  which  enter  the 
epidermal  cells  of  the  host.  There  is  a  method  of  rapid  multi- 
plication during  the  summer  months  by  air  spores,  or  conidia, 


THE  MILDEWS 


249 


which  are  formed  in  chains  from  the  ends  of  erect  hyphse  (Fig. 
219,  A)  and  give  the  leaves  a  powdery  appearance.  But  the 
most  important  fructifications  are  the  sac  fruits  (ascocarps), 
which  appear  later  in  the  season  as  black  dots  on  the  leaves. 
They  can  be  most  conveniently  studied  in  the  lilac  mildew. 

The  lilac  mildew.    This  type  (Microsplicera  Alni)  forms  white 
blotches  on  the  leaves  of  the  lilac,  especially  over  somewhat 


FIG.  219.   Reproductive  organs  of  the  mildews 

A,  B,  the  lilac  mildew  (Microsphxra  Alni) :  A,  a  chain  of  air  spores  (conidia) 
formed  from  the  tip  of  an  erect  filament;  B,  a  sac  fruit  (ascocarp)  cracked 
open,  with  two  spore  sacs  (asci)  protruding,  one  of  the  appendages  shown 
in  detail.  C,  I)  (Podosphsera) :  C,  the  sexual  organs,  —  a  the  antheridium,  b 
the  female  gamete  or  ascogonium;  1),  the  development  of  the  cellular  envel- 
ope of  the  sac  fruit  around  the  fertilized  female  gamete.  —  C,  D,  after  Harper 

shaded  portions  of  the  plant.  The  sac  fruits  are  found  in  the 
autumn  as  black  globular  bodies  made  up  of  filaments  so  closely 
united  that  they  form  a  cellular  mass  (Fig.  219,  B),  in  the 
interior  of  which  are  developed  the  spore  sacs  (asci).  The  sac 
fruit  of  Microsphcera  has  several  radiating  appendages  with 
peculiar  tips.  It  is  developed  as  the  result  of  a  sexual  process 
involving  the  fusion  of  two  sexual  cells,  or  gametes  (Fig.  219,  C), 
The  asci  are  formed  at  the  ends  of  hyphse  that  arise  from  the 


250 


THE  FUNGI 


fertilized  female  cell,  while  the  wall  of  the  ascocarp  is  formed 
from  neighboring  filaments  (Fig.  219,  D).  The  ascocarp  thus 
resembles  in  its  development  the  sexually  formed  fructification 
(cystocarp)  of  certain  red  algse  such  as  Polysiplwnia  (Sec.  245). 
The  ascocarp,  like  the  cystocarp,  is  a  system  of  .two  tissues,  one 
derived  from  the  fertilized  female  gamete  (called  an  ascogonium) 
and  the  other  from  the  vegetative  cells  of  the  parent  plant. 
The  phase  in  the  life  history  beginning  with  the  fertilized  asco- 
gonium and  ending  with  the  production  of  ascospores  is  an 
asexual  or  sporophyte  generation  alternating  with  the  sexual 

generation  or  gametophyte,  as  in  the 
red  alga3  (Sec.  246).  The  wall  of  the 
sac  fruit  is  clearly  a  protective  struc- 
ture for  the  sacs,  each  of  which  gen- 
erally develops  six  spores  in  the  lilac 
mildew,  although  eight  nuclei  are 
present  in  the  sac. 

The  green  and  yellow  mildews. 
These  are  very  common  saprophytes 
on  bread,  cheese,  shoes,  clothing,  and 
other  substances  that  mildew  or 
"mold"  in  dampness.  They  are 
easily  distinguished  by  their  colors 
and  the  structure  of  the  conidial 
fructifications.  The  green  mildew  is 
Penicillium  (Fig.  220,  A),  which  is  believed  to  give  the  peculiar 
flavor  to  Eoquefort  cheese.  The  yellow  mildew  is  Aspergillus 
(Fig.  220,  B).  Their  ascocarps  are  rather  uncommon,  especially 
those  of  Penicilliiim. 

267.  The  cup  fungi.  Most  of  the  conspicuous  forms  in  this 
very  large  assemblage  belong  to  the  order  Pezizales.  The  sac 
fruits  are  saucer--,  cup-,  or  funnel-shaped  (Fig.  221,  J,  J5),  fre- 
quently colored  yellow,  orange,  red,  brown,  or  bluish,  and  in  some 
forms  are  three  or  more  inches  in  diameter.  The  cup  fungi  are 
almost  all  saprophytes,  and  are  found  on  rotten  logs  and  earth  in 


FIG.  220.   Green  and  yellow 

mildews 

A,  the  green  mildew  (Penicil- 
lium) ;  K,  the  yellow  mildew 
(Aspergillus) 


TIIE  CUP  FUNGI 


251 


damp  woods,  forming  very  striking  and  beautiful  growths.  The 
chief  peculiarity  of  the  ascocarps  is  the  fact  that  the  entire 
inner  surface  of  the  cup  is  a  fruiting  surface,  consisting  of  im- 
mense numbers  of  asci,  arranged  upright  and  all  parallel  with 
one  another,  among  delicate  sterile  filaments  (Fig.  221,  C).  The 


FIG.  221.    Cup  fungi 

A,  Lachnea,  a  small  hairy  form  frequently  growing  on  wood ;  B,  Peziza,  a  large 
form  growing  on  earth ;  C,  section  through  the  fruiting  surface  of  a  Peziza 
type,  showing  asci  in  various  stages  of  development  among  delicate  sterile 
filaments  (paraphyses) 

asci  are  thus  exposed,  imbedded  in  a  fruiting  surface,  and  are 
not  inclosed  in  a  case,  as  in  the  mildews. 

The  sac  fruits  of  some  cup  fungi  (notably  Pyronema)  are 
known  to  be  developed  as  the  result  of  a  sexual  process,  but 
there  is  probably  a  great  deal  of  sexual  degeneration  in  this 
group  of  the  fungi,  as  in  the  water  molds  (Sec.  262). 

The  morel.  Some  very  striking  large  forms  are  closely  related 
to  the  cup  fungi.  Among  them  is  the  morel  (Morchella),  much 
prized  as  one  of  the  best  of  the  edible  fungi  (Fig.  222),  and  some 
other  curiously  shaped  types  (Helvella,  Mitrula,  Geoglossum,  etc.). 


252 


THE   FUNGI 


The  fruiting  surface  of  the  ascocarps  is 
sometimes  very  extensive,  and  is  thrown 
up  into  irregular  lobes  and  ridges. 

268.  The  knot  and  wart  fungi.  This 
large  group  contains  forms  with  peculiar 
hard  black  or  brown  wart  and  scab-like 
fructifications,  which  are  found  on  the  bark 
of  trees.  Most  of  the  species  are  sapro- 
phytic,  but  some,  as  the 
black  knot  (Fig.  223),  on 
the  plum  and  cherry,  are 
very  destructive  parasites. 
The  outer  parts  of  the  sac 
fruits  contain  immense 
numbers  of  small  cavities 
(perithecia)  that  are  lined 

Thft  convoluted   upper    with    asci-     VeiT    little    is 

portion  is  an  exposed  known  of  the  development 

fruiting  surface  PI  i  <•      •, 

of  such  complex  sac  fruits, 
but  it  is  probable  that  many  of  these  fungi 
are  sexually  degenerate,  as  are  some  of  the 
cup  fungi.  Xylaria,  with  its  large  finger-like 
fructifications,  belongs  to  this  group. 

269.  Other  sac  fungi.    Several  exceptional 
sac  fungi  deserve  special  mention. 

Ergot.    Ergot  grains  (Fig.  224,  A)  are  hard  FlG  223    The  black 
black  structures  found   in  heads  of  barley, 
rye,  wheat,  and  certain  grasses,  notably  the 
wild  rice.    They  are  really  the  mummified 


FIG.  222.  The  morel 
(Morchella),  an  edi- 
ble sac  fungus 


knot  (Plowrightia) 
on  a  branch  of  the 
cherry 


,     , .  .  .  ,  .        The  branches  become 

and  distorted  ovaries,  or  grains  whose   tis-      distorted,  and  long 
sues  have  become   filled  and  destroyed  by      cracks  are  formed, 

.1  ,.  P  ,,        P  /^VT       .  m-i  greatly  impairing 

the  mycelium  of  the  fungus  (Claviceps).  The      the  strength  of  the 
ergot  represents  a  sort  of  resting  stage  in     trees 
the  life  history  of  the  fungus,  and  from  it  are  developed  in  the 
spring  purplish  stalks  bearing  the  sac  fruits  (Fig.  224,  B). 


OTHER  SAC   FUNGI 


253 


The  caterpillar  and  grub  fungi.  These  extraordinary  parasites 
(Cordyceps)  grow  in  the  bodies  of  certain  caterpillars  and  other 
larvae,  and  in  their  pupae.  The  body  cavity  of 
the  insect  becomes  filled  with  the  mycelium, 
and  generally  mummified,  after  which  a  long- 
stalked  sac  fruit  grows  out  from  between  cer- 
tain segments  (Fig.  225). 

The  truffles.  The  truffles  are  very  remark- 
able sac  fruits,  sometimes  as  large  as  pota- 
toes, which  are  developed  on  my- 
celium that  is  generally  associated 
with  the  roots  of  certain  trees.  The 
commonest  truffle  on  the  market 
(Tuber  Irumale)  comes  from  the  re- 
gion of  Perigord,  in  central  France, 
and  is  the  most  prized  of  all  the  edi- 
ble fungi.  It  grows  under  certain 
kinds  of  oak  trees,  and  is  found  by 
dogs  and  swine  that  are  trained  to 
discover  its  location,  and  which  detect 
the  fungus  by  a  characteristic  odor. 
The  association  of  the  mycelium  of 
the  truffle  with  the  roots  of 
the  oak  tree  is  an  excellent 
example  of  what  is  called 
a  mycorrhiza,  and  is  dis- 

A,  ergot  grains  on  a 

head  of  barley;  13,  CUSSed  in  Sec.  278. 
small  sac  fruits  (as-          ^          t  j         •     ^       t  Flo.  225 

cocarps)   developing  r      J       y 

from  an  ergot  grain.  The  Spot  diseases  of  plants   Caterpillar    fungus 


larva  of  the  May 
beetle,  which  lives 

underround 


upon  the  leaves  and  fruit.    Many  of  them  are 

caused  by  sac  fungi,  as  the  strawberry-leaf  spot 

(Sphcerella),  black  spots  on  grasses  and  clover 

(Phyllachora)  resembling  rust  spots,  tar  spots  on  willow  and 

maple  (Rhytisma),  and  the  apple  scab  (Venturia).    Some  of  the 


254  THE   FUNGI 

most  destructive  rots  are  sac  fungi,  though  frequently  caused 
by  some  kind  of  conidial  fructification  rather  than  by  the  sac 
fruit.  Among  them  are  the  bitter  rot  of  apples  (Glomerella], 
brown  rot  of  peaches  and  plums  (Sclerotinia),  and  plum  pockets 
(Exoascus). 

270.  The  imperfect  fungi.    Some  other  spot  diseases  and  rots 
are  caused  by  fungi  which  are  known  only  through  conidial 
or  other  types  of  asexual  fructification.    More  species  of  these 
forms  have  been  described  than  of  all  the  sac  fungi  together, 
and  they  are  assembled  in  a  group  called  the  Fungi  imperfccti. 
Some  of  them  are  very  important  economically,  causing  such 
diseases  as  the  potato  scab  (Oospora),  tar  spots  (anthracnose)  on 
beans  (Colletotrichum),  black  rot  of  tomato  (Macrosporium),  and 
black  rot  of  apples  (Spliccropsis).    Most  of  the  imperfect  funyi, 
however,  are   saprophytes,   and  play    an  important  part  with 
other  saprophytic  fungi  in  bringing  about  the  decay  of  vege- 
table debris. 

271.  The  lichens.    The  lichens  deserve  special  consideration 
as  a  very  remarkable  group.    They  are  not  single  plants,  but 
composite  organisms  made  up  of  algae  which  are  contained  in 
an  enveloping  mesh  of  fungal  filaments.    The  algal  cells  show- 
ing through  the  fungal  layers    frequently  give    the   lichen   a 
greenish  color,  but  other  pigments  may  be  present,  and  some 
lichens  have   brilliant   yellow,  orange,  brownish,  and  reddish 
tints.    Lichens  have  a  great  variety  of  forms.    Some  grow  closely 
pressed  against  rocks  and  tree  trunks  (crustaceans,  Plate  V,  A), 
some  are  leaf -like  (foliose,  Fig.  226),  and  some  are  much  branched 
(fruticose,  Fig.  227). 

The  fructification  of  a  lichen  is  most  commonly  a  saucer- 
or  cup-shaped  structure.  The  inner  surface  is  a  fruiting  layer 
(Plate  V,  B),  and  contains  numerous  eight-spored  sacs,  or  asci 
(Plate  V,  D),  showing  clearly  that  the  fungi  concerned  in  the 
lichen  are  sac  fungi,  or  Ascomycetcs.  The  fructifications  are 
therefore  sac  fruits,  or  ascocarps,  and  these  are  known  in  some 
forms  to  develop  as  the  result  of  a  sexual  process.  Most  of  the 


PLATE  V.    A  Common  Tree  Lichen  (Physcia  stellaris} 

A,  habit  sketch  ;  B,  diagram  of  a  section  through  a  sac  fruit  (ascocarp),  showing 
the  fruiting  surface  and  layer  of  algal  cells;  C,  section  showing  a  group  of 
algal  cells  (Pleurococcus),  held  in  the  network  of  fungal  filaments;  I),  section 
of  the  fruiting  surface,  showing  sacs  (asci)  in  stages  of  development  among 
the  sterile  filaments  (paraphyses) 


THE  LICHENS  255 

lichens  have  sac  fruits  closely  resembling  those  of  the  cup 
fungi.  There  is  one  small  group  of  tropical  lichens  whose 
fungal  portions  are  basidia  fungi,  or  Basidiomycctes,  and  not 
Ascomycetes. 

The  algal  portions  of  a  lichen  may  be  scattered,  but  in  some 
types  they  are  arranged  in  definite  layers.  The  kinds  of  algae 
differ  in  various  lichens.  Some  of  them  are  unicellular  green 
forms,  evidently  of  the  genus  Pleurococcus  (Plate  V,  C).  Most 
of  the  species  belong  to  the  blue-green  algae,  one-celled  forms 
being  commonest,  though  some  complicated  filamentous  types, 
such  as  Nostoc,  are  found  in  certain  lichens.  One  curious  lichen, 
which  grows  011  the  leaves  of  the  coffee  plant,  contains  a  species 
of  Coleochccte  (Sec.  222). 

The  development  of  the  present  clear  understanding  of  the 
composite,  or  fungal  and  algal,  nature  of  lichens  makes  one  of 
the  most  interesting  chapters  in  the  history  of  botanical  science. 
First  came  the  recognition  of  the  colorless  portion  of  the  lichen 
as  fungal  and  the  colored  elements  as  algal  in  character.  Then 
these  portions  were  separated  and  cultivated  independently  of 
one  another,  which  proved  that  they  remained  respectively  algae 
and  fungi ;  for  example,  the  lichen  spore  never  developed  into 
algal  cells,  but  only  into  fungal  filaments.  Finally,  lichens  were 
created  by  bringing  germinating  spores  in  contact  with  wild 
algae  of  a  suitable  kind,  and  these  lichens  have  in  some  cases 
lived  for  many  months,  finally  developing  typical  lichen  sac 
fruits  (ascocarps),  thus  completing  the  life  history. 

The  lichens  are  perhaps  chiefly  interesting  for  the  relations 
which  the  algae  and  fungi  bear  to  one  another.  When  two 
organisms  live  in  intimate  physiological  association,  so  that 
both  receive  some  benefit  from  the  partnership,  the  condition 
is  called  symbiosis  (meaning  a  living  together).  The  mycorrhiza 
relationship  (Sec.  278)  is  an  excellent  illustration  of  symbiosis. 
It  is  not  easy  to  analyze  critically  the  relationships  between 
the  algae  and  fungi  in  a  lichen  association,  but  some  points 
seem  clear. 


256 


THE   FUNGI 


First.  The  fungi  are  absolutely  dependent  upon  the  algae  for 
their  organic  food  (such  as  the  carbohydrates),  which,  of  course, 
the  algae  are  able  to  manufacture  in  the  manner  characteristic 
of  green  plants  (photosynthesis).  The  relation  of  the  fungus  to 
the  alga  is  then  in  all  essentials  that  of  a  parasite  to  its  host. 

Second.  The  algae  receive  a  certain 
sort  of  protection  in  the  lichen  thallus. 
Thus  they  have  fixed  positions  on  ex- 
posed rocks,  cliffs,  trees,  and  other 
objects  where  they  could  hardly  grow 
otherwise,  or  at  least  not  in  the  same 
luxuriance.  The  substance  of  the  lichen 
also  retains  moisture,  so  that  the  algal 
cells  are  not  so  subject  to  drought. 

It  is  well  known  that  many  of  the 
lowly  algae  would  grow  in  situations 
frequented  by  lichens  if  left  alone,  and 
it  is  evident  that  the  lichens  arise  be- 
cause fungus  spores  fall  among  the  algae, 
and  germinating  produce  hyplue  which 
live  parasitically  upon  them  as  hosts. 

The  algae  are  then,  in  a  sense,  slaves  of 
FIG.  226.   A  leaf-like,  or  fo-   the  f m    {     Th       are  not  killed  for  tha(. 

Hose,  lichen  (Cetraria)  ,        J,  '. 

would  be  oi  no  advantage  to  the  fungus, 

s,  sac  fruits  ...  «          .  i  P 

which  requires  them  to  manufacture 

its  organic  foods.    The  term  slavery  perhaps  best  expresses  the 
relation  of  the  algae  to  the  fungi  in  the  lichens. 

Life  habits  of  the  lichens.  Lichens  are  found  on  rocks,  cliffs, 
branches  and  trunks  of  trees,  and  on  the  ground,  when  the  latter 
cannot  support  green  vegetation,  either  because  it  is  too  bar- 
ren, or  is  exposed  to  unfavorable  climatic  conditions.  They  are 
most  luxuriant  in  temperate  and  sub-arctic  regions,  especially 
where  there  is  much  rain.  They  form  the  bulk  of  the  vegeta- 
tion on  the  tops  of  mountains  and  in  the  arctics,  where  grass 
and  other  alpine  seed  plants  cannot  grow.  They  are  abundant 


SUMMARY  OF   THE  SAC  FUNGI 


257 


along  storm-swept  seacoasts.  Some  forms  actually  cover  large 
areas,  as  the  reindeer  moss  (Cladonia  rangiferina,  Fig.  227,  A), 
which  in  extreme  northern  countries  furnishes  an  important 
source  of  food  for  herbivorous  animals,  as  the  reindeer.  Since 
the  lichens  are  the  first  plants  to  grow  on  exposed  rocks,  they 
form  there  the  first  soil,  mingled  with  decayed  vegetable  matter 
(humus),  which  may  furnish  a  foothold  for  higher  plants,  such  as 
the  mosses  and  grasses, 
that  are  constantly  try- 
ing to  establish  them- 
selves in  the  territory  of 
the  lichens. 

Some  uses  of  lichens. 
Some  lichens  (Roccella) 
yield  beautiful  purple, 
blue,  and  crimson  dyes 
called  orchil  and  cud- 
bear, much  used  in 
former  centuries  in 
Italy,  and  later  in  other 
parts  of  Europe.  Orchil 
when  prepared  with 
soda  or  potash  yields 
the  dye  litmus,  em- 

-i          i    •       ,1  P          A,  the  reindeer   moss   (Cladonia  rangiferina) ; 

ployed  in  the  manufac-       B>  Cladoniacornucopioides-  C,  Usneabarbata; 

ture   of  litmus   paper,      s,  sac  fruits 

Other  lichens,  as  Iceland  moss  (Cetraria),  are  ground  up  and 

mixed  with  wheat  and  made  into  cakes. 

272.  Summary  of  the  sac  fungi.  The  most  remarkable  fea- 
ture of  the  life  history  of  the  Ascomycetes  is  the  position  of  the 
ascocarp  as  a  sporophytic  phase  following  the  sexual  process 
and  alternating  with  sexual  plants,  or  gametophytes.  The  asco- 
carp holds  a  place  in  the  life  history  similar  to  that  of  the 
cystocarp  in  the  red  algse  (Sec.  246).  There  are  numerous  types 
of  asexual  spores  (such  as  conidia)  in  the  Ascomycetes,  which 


FIG.  227.   Some  branching,  or  fruticose, 
lichens 


258  THE   FUNGI 

cannot  be  described  here  but  greatly  complicate  the  classification 
of  the  forms.  Some  authors  believe  that  the  sac  fungi  hold  rela- 
tions to  the  red  algse,  and,  indeed,  have  been  derived  from  them. 

CLASS  IX.    THE  BASIDIA  FUNGI,   OR 
BA  SIDIOMYCETES 

273.  The  basidia  fungi.  The  Basidiomycetes  come  next  to  the 
Ascomycetes  in  number  of  known  species,  which  is  about  fourteen 
thousand.  The  group  takes  its  name  from  a  peculiar  type  of 
reproductive  organ  called  a  basidium  (meaning  a  small  pedestal). 
The  basidium  (Fig.  238)  is  a  somewhat  swollen  terminal  cell 
of  a  filament,  or  hypha,  from  which  are  developed  a  group  of 
four  spores  on  delicate  stalks  called  sterigmata.  The  hyphse  of 
the  basidia  fungi  are  divided  into  cells,  as  in  the  sac  fungi. 

The  basidium  is  a  very  characteristic  structure  of  the  higher 
forms  of  the  Basidiomycetes.  However^  there  are  some  types,  as 
the  smuts  and  rusts,  in  which  the  basidium  is  represented  by  a 
peculiar  phase  in  the  life  history  (the  promycelium),  which  does 
not  at  first  thought  seem  to  resemble  the  basidium.  These 
points  can  only  be  made  clear  after  a  study  of  representative 
types,  and  they  will  be  referred  to  later  in  the  summary  of  the 
basidia  fungi  (Sec.  279).  This  peculiarity  is  the  basis  of  a  classi- 
fication of  the  basidia  fungi  into  two  series:  (1)  the  Protobasid- 
iomycetes,  which  are  preliminary  to  (2)  the  Eukasidiomycetes, 
or  typical  basidia  fungi.  The  representatives  that  can  be  con- 
sidered here  will  accordingly  be  grouped  as  follows : 

SERIES  I.    The  simpler  basidia  fungi,  or  Protobasidlomycetes. 

1.  The  smuts,  or  Ustilaginales. 

2.  The  rusts,  or  Uredinales. 

SERIES  II.  The  typical  basidia  fungi,  or  Eubasidiomycetes. 

3.  The  coral  fungi,  the  pore  fungi,  the  tooth  fungi,  the  gill  fungi,  col- 

lectively called  Hymenomycetes;  and  divided  into  several  orders. 

4.  The  puffballs,  the  earth  stars,  the  nest  fungi,  the  carrion  fungi, 

collectively  called  Gastromycetes,  and  divided  into  several  orders. 


THE   SMUTS  259 

SERIES  I.  THE  SIMPLER  BASIDIA  FUNGI,  OR 

Pli  O  TOBAS  IDIOM  YCE  TES 

274.  The  smuts.  The  smuts  (order  Ustilaginales)  are  para- 
sites which  have  the  peculiar  habit  of  attacking  the  floral  parts, 
and  especially  the  ovaries,  of  various  members  of  the  grass 
family.  The  hyphae  fill  these  parts  with  a  dense  mycelium, 
destroying  the  tissue  of  the  host.  Finally,  most  of  the  cells  in 
the  mycelium  take  on  heavy  walls  and  become 
resting  cells,  or  winter  spores,  which  form  the 
black  powdery  mass  so  characteristic  of  the 
smut  fructification.  These  resting  cells  sur- 
vive the  winter  and  germinate  in  the  spring. 
Each  cell  then  puts  forth  a  short  filament 
called  the  promycelium  (Fig.  228,  A),  upon 
which  are  developed  a  number  of  small  spring 
spores  called  sporidia,  and  these  in  some  cases 
germinate  upon  the  sprouting  host  plants,  as  FJG  228.  Promyce- 
in  oats,  putting  forth  filaments  that  enter  the  liuui  of  the  corn 


host  and  develop  a  mycelium  within,  which  smut    ( 
may  not  be  noticed  until  the  fructifications 

,-,        n        i                       -f.     .      .  A.  with  spriner  spores 

appear  in  the  floral  organs.    It  is  important  (sporidia)  attached  ; 

to  note  that  the  sporidia  multiply  rapidly  by  ^>    spring   spores 

,      ,  ,.         /T7.      ooo     -ox                •    n            j       *  budding  like  yeast 

budding  (Fig.  228,  B),  especially  under  favor-  ceils.  —  After 


able  conditions,  as  in  heavily  manured  soils, 
and  these  buds,  or  conidia,  will  infect  like  the  sporidia.  These 
habits  of  budding  led  to  the  theory  that  the  yeasts  have  been 
derived  from  the  smuts. 

Various  smuts.  The  corn  smut  is,  perhaps,  the  most  con- 
spicuous form  and  very  destructive.  The  infection  in  the  corn 
is  local  ;  that  is,  the  spore  masses  are  formed  close  to  the  point 
of  entrance  of  the  fungus.  Any  tender  growing  region  is  sub- 
ject to  infection.  The  corn  smut  can  only  be  held  in  check  by 
burning  the  spore  masses  as  soon  as  discovered  and  by  avoid- 
ing the  use  of  manure,  which  gives  favorable  nutrition  for  the 


260 


THE  FUNGI 


germination  of  the  spores.  The  smuts  of  oats  and  wheat  often 
cause  enormous  loss  in  these  crops.  The  best  preventive  measures 
seem  to  be,  treatment  of  the  grains  with  solutions  of  copper  sul- 
phate, or  formalin,  or  steeping  them  in  hot  water  for  a  short  time 
before  planting,  which  kills  the  smut 
spores  without  injuring  the  grain.1 

275.  The  rusts.    The  rusts  (order 
Uredinales)    cause    some  of   the   most 


FIG.  229.  The  wheat 
rust  (Puccinia  gram- 
inis) 

A,  spots  of  the  red  rust 
on  a  wheat  leaf,  com- 
posed of  the  summer 
spores  (uredospores) ; 
H,  spots  of  the  black 
rust  on  wheat  straw, 
composed  of  the 
winter  spores  (teleuto- 
spores) 


FIG.  230.  The  winter  spores  (teleuto- 
spores)  of  the  wheat  rust  (Puccinia 
graminis) 

Section  through  a  spot  of  the  black  rust  on 
oats,  the  epidermis  of  the  leaf  being 
thrown  back  and  the  two-celled  teleuto- 
spores  raised  above  the  surface  on  stalks ; 
note  the  web  of  fungal  filaments  (hyphre) 
around  the  very  much  enlarged  (hyper- 
trophied)  cells  of  the  host  under  the  spot 


disastrous  diseases  of  such  grains  as  wheat,  oats,  barley,  and 
rye.    They  are  all  parasites,  forming  yellow  or  black  spots  on 

1  See  Swingle,  "  The  Prevention  of  Stinking  Smut  of  Wheat  and  Loose 
Smut  of  Oats,"  United  States  Department  of  Agriculture,  Farmer's  Bulletin 
250,  1906. 


THE  RUSTS 


261 


the  leaves  and  stems  of  their  hosts.  The  most  complicated  life 
histories  in  the  fungi  are  found  in  this  group,  for  many  spe- 
cies require  two  different  hosts  to  complete  their  life  cycle  and 
form  a  number  of  different  reproductive  spores  during  their 
development.  These  peculiarities  are  best  illustrated  by  the 
rust  of  wheat. 

The  wheat  rust.  The  wheat  rust  (Puccinia  graminis)  appears 
on  wheat,  oats,  and  other  grains  and  grasses,  first  as  red  or 
yellow  streaks  or  spots  upon  the  leaves  and 
stems  (Fig.  229,  A).  The  host  is  greatly  weak- 
ened and  consequently  matures  only  a  small 
yield  of  grain.  Towards  the  end  of  the  season 
black  streaks  (Fig.  229,  B)  are  formed  in  ad- 
dition to  the  red-rust  spots,  and  these  indicate 
the  development  of  resting  cells,  or  winter 
spores,  which  are  peculiar  two-celled  struc- 
tures in  Puccinia  (Fig.  230).  The  winter 
spores,  called  teleutospores,  germinate  in  the 
spring,  and  each  cell  gives  rise  to  a  short 
filament,  the  promycelium,  usually  consisting 
of  four  cells  (Fig.  231),  from  which  are  gen- 
erally developed  four  spring  spores,  or  sporidia. 
The  winter  spores,  promycelium,  and  spring 
spores  probably  correspond  to  the  same  stages 
in  the  life  history  of  a  smut. 

Wherever  the  barberry  is  common,  as  in 

Europe  and  New  England,  the  spring  spores  *io.  231.  Promyce- 

7  1mm  of  the  wheat 

(sporidia)  infect  these  plants  and  produce  on      rust  (Puccinia 

their  leaves  peculiar  fructifications  called      graminis) 
cluster  cups,  or  cecidia   (Fig.  232,  A,  B),  in       After  Tuiasne 
which  are  developed  chains  of  cluster-cup  spores,  or  cccidiospores 
(Fig.  232,  C).    There  is   considerable   evidence  to   prove   that 
the  cluster  cups  represent  the  remains  of  what  was  once  a 
sexual  phase  in  the  life  history  of  the  rust,  but  which  is  now 
much  modified,  and  indeed  entirely  suppressed  in  some  forms. 


262 


THE   FUNGI 


Curious  structures  called  spermogonia  (Fig.  232,  C)  frequently 
accompany  the  cluster  cups  and  are  believed  to  be  the  remains 
of  male  sexual  organs  now  no  longer  functional.  They  develop 
immense  numbers  of  minute  cells,  termed  spermatia,  which  may 
at  one  time  have  been  functional  sperms,  but  apparently  serve 
no  useful  purpose  now. 

The  secidiospores  are  distributed  by  the  wind  and  germinate 
upon  young  wheat,  putting  forth  tubes  which  enter  the  host 


FIG.  232.   Cluster  cups  (secidia)  on  barberry  leaves 

A,  habit  sketch  showing  groups  of  cluster  cups  on  a  leaf;  B,  a  group  enlarged ; 
C,  section  through  a  leaf  showing  cluster  cups  on  the  lower  surface,  with  the 
chains  of  secidiospores  and  the  male  organs  (spermogonia)  on  the  upper  sur- 
face. The  latter  develop  immense  numbers  of  minute  cells  which  probably 
represent  sperms,  but  are  now  functionless 

through  the  stomata.  The  infected  wheat  then  develops  a 
number  of  crops  of  one-celled  summer  spores  called  uredo- 
spores  (Fig.  233).  The  first  crops  of  summer  spores  are  widely 
scattered  in  high  winds  and  infect  more  wheat,  thus  spreading 
the  disease  very  rapidly.  The  spots  of  uredospores  are  reddish 
or  yellowish,  and  this  is  the  stage  known  as  the  red  rust  of 
wheat.  Finally,  at  the  end  of  the  season,  the  black  spots  of 
teleutospores  appear,  and  the  rust's  life  history  is  completed. 


THE  RUSTS 


263 


This  long  life  history,  which  is  thoroughly  known  in  Europe, 
becomes  much  shortened  in  the  Middle  West,  California,  and 
Australia,  where  there  is  no  barberry,  by  the  omission  from 
it  of  that  host.  In  these  regions  the  uredospores  (summer 
spores)  may  survive  the  winter  or 
dry  season,  or  be  carried  over  from 
summer  to  summer  through  the 
winter  wheat  and  germinate  di- 
rectly upon  the  new  developing  wheat 
of  the  following  year,  so  that  the  re- 
production of  the  rust  is  by  a  succes- 
sion of  the  uredospores. 

There  is  no  method  known  of  killing 
the  wheat  rust  on  the  living  host ;  but 
it  has  been  found  that  certain  varie- 
ties of  wheat,  as  the  macaroni  wheats, 
are  far  more  resistant  to  the  rust  than 
others.  There  is  some  hope  that  varie- 
ties may  be  bred  by  crossing  our  wheats 
with  macaroni  wheat  that  will  be 
largely  immune  to  this  disease,  which 
annually  causes  losses  of  many  million  FlG-  233-  The  summer  spores 
dollars  in  the  United  States  alone.  1  £tS3  ^  ^  "* 
There  are  a  large  number  of  varieties  A  single  twolLteieutospore, 
of  Puccinia  graminis,  and  also  several  t,  happens  to  be  presentamong 
other  species  of  Puccinia  which  attack  them'  ~  After  De  Bary 
various  grains,  grasses,  and  other  plants.  One  of  these  (P.  as- 
paragi)  sometimes  causes  great  damage  to  asparagus. 

Other  rusts.  The  group  of  the  rusts  is  very  large,  the  genera 
being  distinguished  chiefly  by  the  structure  of  the  teleutospores 

1  For  a  discussion  of  the  rusts  and  rust  problems  of  the  United  States,  see 
papers  of  Carleton  from  the  publications  of  the  United  States  Department  of 
Agriculture,  "Cereal  Rusts  of  the  United  States,"  Division  of  Vegetable 
Physiology  and  Pathology,  Bulletin  16,  1899  ;  "  Macaroni  Wheats,"  Bureau 
of  Plant  Industry,  Bulletin  3,  1901;  "Investigations  of  Rust,"  Bureau  of 
Plant  Industry,  Bulletin  63,  1904. 


264 


THE  FUNGI 


and  the  different  types  of  life  histories  affecting  various  hosts; 
but  many  of  the  forms  have  no  economic  importance,  being 
found  on  such  plants  as  the  violet,  May  apple,  cocklebur, 
asters,  golden-rods,  members  of  the  pea  family,  etc.  However, 
there  are  destructive  rusts  on  the  roses  (Phragmidium),  clovers 
(Uromyces],  blackberries  (Ceoma),  etc.  An  interesting  type  is 
the  rust  (Gymnosporangium,)  which  causes  the  distortions  called 
cedar  apples  on  the  junipers,  and  the  much-branched  stunted 
growths  called  witches'  brooms.  This  rust  has  a  cluster-cup 
stage  (once  named  Rwstelia)  on  the  hawthorn  and  apple. 


SERIES  II.  THE  TYPICAL  BASIDIA  FUNGI,  OR  EUBASIDIOMYCETES 

276.  The  Hymenomycetes.  This  group,  which  may  be  con- 
sidered a  sub-class  of  the  Basidiomycetes,  comprises  all  of  the 
higher  basidia  fungi  whose  spores  are 
developed  on  a  fruiting  surface,  called 
an  liymenium  (meaning  a  membrane), 
which  is  exposed.  This  condition  is 
thus  contrasted  with  that  in  the  Gas- 
tromycetes  (puffballs,  etc.),  where  the 
spores  are  developed  within  a  case.  The 
types  of  fructification  are  exceedingly 
various  in  this  group,  which  includes 
the  pore,  the  tooth,  and  the  gill  fungi 
in  the  various  forms  of  toadstools  and 
brackets.  But  there  are  also  some 
simpler  types,  as  the  coral  fungus  (Cla- 
varia),  with  irregular  branches  (Fig. 
234),  and  also  some  expanded  forms. 
The  pore  fungi.-  The  pore  fungi 
(family  Polyporacece)  have  commonly  the  shape  of  brackets  and 
grow  on  the  trunks  of  trees,  although  some  are  large,  fleshy  toad- 
stools, as  Boletus  (Fig.  235).  The  hymenium  lines  the  cavities 
of  the  numerous  pores  which  are  found  on  the  under  surfaces. 


FIG.  234.   A  coral  fungus 
(Clavarid) 


THE   IIYMENOMYCETES 


265 


Many  of  the  pore  fungi  are  perennial,  increasing  in  size  from 

year  to  year  by  adding  new 
layers  of  growth  outside  of 
the  old.  The  bracket  or  toad- 
stool is  merely  the  fructifica- 
tion which  receives  its  nour- 
ishment from  an  extensive 
mycelium  growing  in  the 
wood,  and  under  the  bark  of 
trees,  or  in  the  soil.  Many 
of  the  pore  fungi  are  very 
destructive  parasites,  greatly 
injuring  and  sometimes  kill- 
ing forest  trees.  They  may 
cause  great  injury  to  growing 
timber.1  Most  of  the  pore 
fungi  are,  however,  sapro- 
phytic  in  their  manner  of 
life. 

The  tooth  fungi.  The  tooth  fungi  (family  Hydnacece)  are  less 
common  than  the  pore  and 
gill  fungi.  Some  of  them  have 
bracket  forms,  and  some  are 
toadstools  (Fig.  236).  The 
fruiting  surface  is  distrib- 
uted over  tooth  or  spine-like 
processes. 

The  gill  fungi.  The  gill 
fungi  (family  Agaricacecc}  in- 
clude most  of  the  toadstool 
and  mushroom  forms  (Fig. 
237).  A  toadstool  consists  of  FlG<  236.  A  tooth  fungus  (Hydnum) 

1  See  von  Schrenk,  "The  Decay  of  Timber  and  Methods  of  Preventing 
It,"  United  States  Department  of  Agriculture,  Bureau  of  Plant  Industry, 
Bulletin  14,  1902. 


FIG.  235.   A  pore-bearing  toadstool 
(Boletus) 


266 


THE  FUNGI 


--cap 


a  stalk  (stipe))  which  in  some  genera  arises  from  a  cup  (volva) 
and  is  expanded  above  into  the  cap  (pileus).    The  under  surface 

of  the  cap  bears  many  thin  plates 
which  hang  down  in  a  radiating  ar- 
rangement and  are  called  gills.  The 
gills  illustrate  very  well  the  struc- 
ture and  position  of  the  basidia  on 
a  fruiting  surface,  or  hymenium, 
and  cross  sections  are  shown  in  Fig. 
238.  It  will  be  seen  that  the  basidia 
are  the  swollen  terminal  cells  of  a 
compact  mesh  of  hyphae,  and  that 
each  bears  a  group  of  four  spores 
on  short  stalks  or  sterigmata. 

The  toadstool  is  really  a  fructi- 
fication. It  is  attached  to  an  ex- 
tensive mass  of  mycelium,  which 
is  the  vegetative  portion  of  the 
plant.  This  mycelium  generally 
lives  saprophytically  in  the  soil, 
frequently  around  buried  roots  of 
trees,  but  there  are  some  para- 
sitic gill  fungi  (Plate  VI)  which 
cause  the  decay  and  final  death  of 
valuable  timber.  The  toadstool 
develops  from  an.  accumulation  of 
hyphse  in  small  structures  called 


FIG.  237.  A  group  of  mushrooms 
(Armillaria  mellea) 


my,  mycelial  attachment ;  c,  cf,  c", 
young  stages  called  buttons; 
mature  mushroom  with  expanded 
cap  (pileus)  shown  ahove;  st, 
stem  (stipe);  g,  gills;  r,  ring. — 
After  Hartig,  through  Bennet 
and  Murray 


buttons    (Fig.  237,  c, 


The 


cap  region  with  the  gills  and  stalk 
become  differentiated  within  the 
button,  and  finally  break  out  from 
the  surrounding  envelope  and  ex- 
pand in  a  few  hours  to  their  full  size ;  hence  the  expression  a 
"  mushroom  growth."  The  remains  of  the  envelope  are  found  in 
some  forms  as  scales  on  the  top  of  the  cap  (see  mature  mushroom 


THE  HYMENOMYCETES 


26T 


of  Fig.  237)  and  in  a  ring  attached  to  the  stalk  below  the  gills 
(Fig.  237,  r),  while  in  certain  types  (Amanita,  etc.)  there  is  a 
large  cup  (volva)  at  the  base  of  the 
plant  out  of  which  the  stalk  rises. 

It  is  becoming  rather  general  popu- 
lar usage  to  apply  the  term  mushroom 
to  all  toadstools  and  other  fleshy 
fungi  which  are  edible.  There  are  no 
general  rules  for  distinguishing  mush- 
rooms from  toadstools  which  do  not 
have  exceptions;  but  the  collector 
may  readily  learn  the  characters  of 
the  most  -poisonous  species,  and  like- 
wise become  acquainted  with  a  num- 
ber of  choice  forms  which  are  easily 
recognized.1  It  is  a  good  principle, 
however,  to  rest  satisfied  with  a 
knowledge  of  a  few  absolutely  safe 
mushrooms  and  not  to  experiment 
with  those  that  are  not  fully  known. 

The  most  poisonous  species  of  the  gill   ^  ««*•.•••'* 

r  FIG.  238.   Gills  of  mushroom 

fungi  are  in  the  genus  Amanita  and     .      (Coprinus  comatus) 

have  large  volvas,  rings,  and  white    Across  section  of  gills  showing 

spores,  and  may  be  readily  recognized      fruiting  surface  (hymenium) ; 

when  carefully  examined.     There  are 

also  some  very  poisonous  species  of 

Boletus  among  the  pore  fungi.    The 

commonest  mushroom  of  the  market 

(Agaricus  campestris)   is   a   form   extensively   cultivated,   but 

which  also  grows  in  the  fields.    These  mushrooms  are  raised 

in  cellars  and  caves,  in  specially  prepared,  heavily  manured 

beds,    which    are    planted    with    masses    of    mycelium   called 

1  See  Farlow,  "  Some  Edible  and  Poisonous  Fungi,"  United  States  Depart- 
ment of  Agriculture,  Division  of  Vegetable  Physiology  and  Pathology,  Bul- 
letin 15,  1898. 


,  portion  of  fruiting  surface 
illustrating  three  basidia  with 
spores  and  two  from  which 
the  spores  have  fallen  off, 
showing  the  spore-bearing 
stalks  (sterigmata)  s 


268 


THE   FUNGI 


spawn.1  Some  species  of  Boletus  are  edible,  and  they,  with  the 
morels  (Sec.  267)  and  truffles  (Sec.  269),  are  sold  in  the  Euro- 
pean markets  with  edible  gill  fungi. 

277.  The  Gastromycetes.  This  group,  in  contrast  with  the 
Hymenomycetes,  includes  forms  in  which  the  basidia  line  the 
interior  of  chambers,  or  cavities,  in  the  fructifications  and  are 
consequently  inclosed  until  the  fructification  matures.  Here 

are  found  the  puffballs,  earth 
stars,  nest  fungi,  and  carrion 
fungi. 

The  puffballs.  These  are 
the  fructifications  (Fig.  239) 
of  an  extensive  underground 
saprophytic  mycelium,  as  in 
the  toadstools  and  mushrooms. 
The  young  puff  ball  has  a 
white  flesh  made  up  of  hyphse 
and  filled  with  small  irregular 
cavities  lined  with  the  fruiting 
FIG.  239.  Apuffball(L^erdon)  gurface  (hvmenium).  The 

spores  when  ripe  lie  freely  as  a  brown  powder  in  the  dried-up 
fibrous  tissue  inclosed  .in  an  outer  parchment-like  envelope. 
The  spores  may  be  discharged  through  a  special  opening  at  the 
top  or  scattered  by  the  irregular  rupture  and  decay  of  the  puff- 
ball.  Young  puffballs  are  edible,  and  there  is  one  extraordinary 
species  (Lycoperdon  giganteum]  which  grows  to  be  a  foot  or 
more  in  diameter  and  is  much  prized  as  a  delicacy. 

The  earth  stars.  The  earth  stars  (Gfcaster,  Fig.  240)  are  modi- 
fied forms  of  puffballs.  The  envelope  is  very  thick,  and  the 
outer  portion  splits  lengthwise  into  segments  which,  when  wet, 
curve  back  from  above  and  raise  the  fructifications  from  the 
ground.  In  dry  weather  the  segments  are  usually  rolled  up 

1  See  Duggar,  "  The  Principles  of  Mushroom  Growing  and  Mushroom 
Spawn  Making,"  United  States  Department  of  Agriculture,  Bureau  of  Plant 
Industry,  Bulletin  85,  1905, 


THE   GASTROMYCETES 


269 


tightly  around  the  fructifications.  These  movements  of  the  seg- 
ments in  certain  species  when  alternately  wet  and  dry  sometimes 

tear  the  earth  stars  loose  from 
the  ground  so  that  they  may 
roll  about,  thus  assisting  in 
the  distribution  of  the  spores. 
The  puffballs  and  earth 
stars  are  in  the  same  order 
(Ly  coper  dales). 

The  nest  fungi.  These  beauti- 
ful little  forms  (order  Nidula- 

riales)  grow  on  the  earth  and 
FIG.  240.   An  earth  star  (Geaster)        decaying  wood  and  when  Qpen 

resemble  a  nest  rilled  with  eggs  (Fig.  241).  The  egg-like  struc- 
tures are  portions  of  the  interior  of  the  fructification,  and  each 
contains  a  chamber  filled  with  spores. 

The  carrion  fungi.  These  very  malodorous  fungi  (order 
Phallalcs)  grow  in  rich  humus  and  mulchings.  They  are  com- 
plicated stalked  types  first  formed  within  a  'large  globular 
structure  which  remains  around  the  base  of  the  stalk  as  a  cup. 
The  top  of  the  stalk  bears  a  dark-colored,  sticky  mass  of  spores, 
that  has  the  odor  of  carrion 
and  attracts  carrion  flies, 
which  probably  assist  in  the 
distribution  of  the  spores. 

278.  Mycorrhiza.  Mycor- 
rhiza  (meaning  fungus-in- 
fected roots)  is  a  remarkable 
association  of  the  mycelium 
of  certain  fungi  with  the 
roots  of  many  seed  plants, 
notably  trees.  The  fungal  filaments  surround  the  roots  with  a 
web  (Fig.  242)  and  enter  the  outer  regions  of  the  root  tissue, 
probably  living  somewhat  parasitically  upon  the  plant  as  a 
host.  They  are  in  close  contact  with  the  soil  around  the  roots, 


FIG.  241.   A  nest  fungus  (Cyathus) 

The  section  at  the  right  shows  the  egg-like 
structures  containing  the  spores 


270 


THE  FUNGI 


and  are  believed  to  be  of  great  assistance  to  them  in  their  work 
in  the  following  way.  It  is  necessary  for  the  roots,  of  course,  to 
establish  a  close  relation  to  the  moisture  of  the  soil  in  order  to 
obtain  water  for  the  green  parts  of  the  plant  above  ground.  The 
surface  of  the  older  portions  is  without  root  hairs  and  is  sur- 
rounded by  a  hard  outer  layer  which  cannot  come  into  very 
close  contact  with  the  minute  moist  soil  particles.  But  it  is 
thought  that  the  fungal  filaments  act  as  root  hairs,  and  perhaps 
through  them  the  root  can  absorb  a  much  greater  quantity  of 

water  and  can  well  afford  to  give 
them  what  nourishment  they  require 
in  exchange  for  such  valuable 
services.  It  is  probable  that  most 
trees  and  many  others  of  the  larger 
plants  have  formed  this  partnership 
with  the  fungi.  The  kinds  of  fungi 
concerned  with  mycorrhizas  are  not 
well  understood,  but  some  of  them 
are  known  to  be  the  mycelia  of 
toadstools  and  puffballs.  The  sac 
fungi  also  furnish  notable  examples 
in  the  truffles  (Sec.  269).  The  my- 
corrhiza  relationship  is  an  excellent 
illustration  of  symbiosis  (which 
means  a  living  together),  for  two 


FIG. 242.  Mycorrhiza  surround- 
ing the  tip  of  a  beech  root 

After  Pfeffer 


organisms  exist  here  in  intimate  physiological  association  and 
both  apparently  receive  benefit  from  the  partnership. 

279.  Summary  of  the  basidia  fungi.  The  relationships 
between  the  different  groups  of  the  Basidiomycetes  cannot  be 
discussed  further  than  to  state  that  the  promycelium  of  smuts 
and  rusts,  with  its  sporidia,  is  believed  to  correspond  to  the 
basidium  with  its  four  spores.  There  are  two  small  groups  called 
the  jelly  fungi  (orders  Auricularales  and  Tremellales),  includ- 
ing the  rather  common  Jew's-ear  fungus,  whose  basidia  become 
divided  into  four  parts.  In  the  Jew's-ear  fungus  the  basidium 


PLATE  VI.   A  wound  parasite  (Pleurotus  ulmarius)  on  the  trunk  of  a 

maple  tree 
After  E.  M.  Freeman 


SUMMARY  OF   THE  BASIDIA  FUNGI  271 

is  indeed  a  four-celled  filament  resembling  the  promycelium  of 
a  rust,  each  cell  developing  a  spore  at  one  side  on  a  sterigmata. 
The  winter  spores,  or  teleutospores,  of  the  smuts  and  rusts  are 
considered  to  be  special  resting  cells  of  the  fungi,  developed  to 
carry  these  parasitic  forms  over  unfavorable  seasons  of  cold  or 
drought  when  the  host  plants  are  not  alive.  There  is  thus  a 
break  in  the  life  history  at  the  point  where  the  basidium  should 
normally  appear.  The  germination  of  these  spores  continues 
the  life  history  with  the  immediate  development  of  a  structure 
(the  promycelium)  which  corresponds  to  a  basidium  with  its 
spores. 

The  higher  basidia  fungi  have  apparently  lost  all  trace  of 
sexual  organs,  but  the  cluster-cup  stage  in  the  rusts  is  believed 
to  indicate  the  remains  of  a  modified  sexual  generation  in  their 
life  histories.  The  evidence  for  this  view  rests  chiefly  upon  the 
behavior  of  the  nuclei  throughout  the  life  history  of  the  rust 
and  is  too  complicated  for  treatment  here.  The  basidia  fungi 
are  therefore  chiefly,  if  not  wholly,  apogamous.  The  origin  and 
evolution  of  the  Basidiomycetes  is  a  problem  as  yet  unsolved, 
which  cannot  be  here  considered.  The  basidia  fungi  are,  how- 
ever, by  far  the  most  wonderfully  varied  and  specialized  assem- 
blage of  the  fungi. 


CHAPTEE   XXIII 

SUMMARY   OF  THE   LIFE   HISTORIES  AND  EVOLUTION 
OF  THE   FUNGI 

280.  The  life  histories  of  the  fungi.  To  understand  the 
types  of  life  histories  in  the  different  groups  of  the  fungi  one 
must  bear  in  mind  the  life  histories  of  the  most  nearly  related 
groups  of  algse  (Sec.  247),  for  those  of  the  fungi  are  based,  of 
course,  on  the  life  histories  of  their  algal  ancestors.  But  there 
have  been  some  very  important  modifications  as  the  result  of 
the  parasitic  and  saprophytic  modes  of  life  of  the  fungi,  and 
especially  because  the  highest  groups  of  fungi  present  much 
sexual  degeneration,  or  apogamy,  which  of  course  in  some 
respects  simplifies  the  life  histories. 

The  life  history  of  the  bacteria  is  essentially  as  simple  as 
that  of  the  blue-green  algse.  The  alga-like  fungi  (Pliy  corny  cdes) 
is  a  group,  however,  whose  highest  members  (the  molds,  water 
molds,  and  blights)  have  reproductive  organs  with  many  points 
of  similarity  to  the  siphon  algre,  and  more  especially  to  Vau- 
cheria  (Sec.  228).  The  sexually  formed  spores  generally  develop 
directly  into  plants  like  the  parents,1  so  that  the  formula  for  the 
life  history  is 


P—  <>sc£.  s.  —  P<>se£.  s.  —  P,  etc., 
^9  9 

the  abbreviations  g  and  sex.  s.  standing  for  gamete  and  sexually 
formed  spore,  respectively.  There  is  often  extensive  reproduc- 
tion through  various  forms  of  asexual  spores  between  succes- 
sive sexual  generations.  And  indeed  sexual  organs  may  only 
be  formed  at  rare  intervals,  as  in  the  bread  mold,  or  they  may 

1  They  form  zoospores,  however,  in  some  of  the  blights. 

272 


THE  LIFE  HISTORIES  OF  THE  FUNGI  273 

not  be  functional  so  that  a  condition  of  apogamy  is  present,  as 
in  the  water  molds. 

The  life  histories  of  the  sac  fungi  (Ascomycetcs)  are  especially 
interesting  in  relation  to  those  of  the  red  algse  (Sec.  246).  It  is 
known  in  regard  to  a  number  of  types  that  the  sac  fruits  (asco- 
carps)  develop  as  the  result  of  a  sexual  process,  corresponding  in 
this  respect  to  the  cystocarps.  The  ascospores  are  formed  at  the 
end  of  the  ascocarp  phase  of  the  life  history  just  as  the  carpo- 
spores  are  formed  at  the  end  of  the  cystocarp  phase  in  the  red 
algse.  Both  ascocarps  and  cystocarps  are,  then,  new  genera- 
tions developed  between  and  alternating  with  the  sexual  plants. 
They  are  sporophytes  alternating  with  gametophytes.  The  for- 
mula for  the  life  history  of  a  sac  fungus  with  functional  sexual 
organs  is  then 

£<;^>  —  S  (ascocarp)  —  asex.  s.  (ascospore) 

u 

—  £ <^ >  —  S—  asex.' s.  —  G,  etc., 

if 

G  and  S  standing  for  garnetophyte  and  sporophyte,  respectively, 
and  asex.  s.  for  asexual  spore. 

It  must  always  be  remembered,  however,  that  the  sac  fungi 
have  a  great  variety  of  methods  of  asexual  reproduction  through 
conidia,  etc.  Consequently  sexual  organs  may  be  formed  only 
occasionally,  as  in  the  green  mildew  (Penicillium).  There  is 
also  probably  much  apogamy  in  the  group,  so  that  the  sac  fruits 
are  apogamously  developed. 

The  basidia  fungi  present  the  remains  of  an  alternation  of 
generations  in  the  rusts  somewhat  similar  to  that  of  the  sac 
fungi.  The  cluster  cups  are  believed  to  be  the  beginning  of  a 
phase  that  formerly  followed  a  sexual  process  just  as  do  the 
ascocarps  and  cystocarps.  However,  the  male  organs  (spermo- 
gonia)  of  the  rusts  are  no  longer  functional,  and  the  cluster  cups 
must  be  considered  as  developing  apogamousy,  although  there 
is  now  a  complicated  history  substituted  for  the  original  sexual 


274     LIFE  HISTORIES  AND  EVOLUTION  OF  THE  FUNGI 

process.  The  cluster-cup  stage  is  omitted  entirely  in  some  of 
the  rusts  and  in  all  of  the  smuts,  and  there  are  likewise  no 
traces  of  it  in  the  higher  basidia  fungi  (Eiibasidiomycetes). 
Sexual  degeneration  in  these  forms,  then,  has  apparently  been 
carried  so  far  that  the  sexual  organs  have  disappeared  entirely 
from  the  life  histories. 

281.  The  origin  and  evolution  of  the  fungi.  The  study  of  the 
evolution  of  the  fungi  must  be  taken  up  for  each  of  the  larger 
classes  separately,  for  there  is  every  probability  that  each  has  had 
an  independent  origin  from  widely  separated  groups  of  the  algre. 
The  bacteria  have  probably  been  derived  from  the  blue-green  algse. 
The  higher  alga-like  fungi  (molds,  water  molds,  and  blights) 
apparently  show  relationships  to  the  siphon  algae.  Some  authors 
believe  that  the  sac  fungi  have  come  from  the  red  algse.  The 
origin  of  the  basidia  fungi  is  very  much  in  doubt  and  that  of 
the  yeasts  also,  although  it  is  generally  held  that  the  latter  are 
degenerate  forms  from  some  of  the  higher  fungi.  The  evolu- 
tion of  the  fungal  forms  in  each  group  becomes  very  complicated, 
because  the  fungi  have  such  wonderfully  varied  habits  resulting 
from  their  parasitic  and  saprophy tic  ways  of  living.  In  fact,  these 
life  habits  have  produced  the  greatest  variety  of  structures  and 
adaptations  known  in  any  group  of  spore  plants.  Still  more 
remarkable,  perhaps,  is  the  widespread  tendency  towards  sexual 
degeneration,  which  is  also  believed  to  be  associated  with  the 
parasitic  and  saprophytic  life  habits. 


CHAPTEK  XXIV 


THE  BRYOPHYTES   AND   THE   ESTABLISHMENT   OF  ALTER- 
NATION OF  GENERATIONS 

282.  The  bryophytes.*  The  division  Bryophyta  (meaning 
moss  plants)  is  the  next  great  group  of  plants  above  the  division 
Tliallopliyta  (Chapter  xix),  and  includes  two  classes:  (1)  the 
liverworts,  or  Hepaticw,  and  (2)  the  mosses, 
or  Musci.  It  is  not  best  to  define  these 
classes  until  the  structure  and  life  histories 
of  types  from  each  group  have  been  studied. 
Furthermore,  it  is  impossible  fully  to  under- 
stand the  characters  of  the  bryophytes  and 
thallophytes  except  when  compared  with 
one  another.  Accordingly  these  matters 
have  been  reserved  for  the  final  section  of 
this  chapter  under  the  heading  Summary 
of  the  Bryophytes  and  Thallophytes  (Sees. 
300,  301). 

However,  the  bryophytes  differ  from  the 
thallophytes  in  two  very  important  respects  Antheridium  in  section, 

showing  the  outer  cap- 

which  may  be  briefly  stated  at  once,  for 

they  must  be  thoroughly  comprehended  in 

order  to  understand  the  life  histories  of  the 

liverworts  and  mosses.    They  can  only  be 

made  clear  when  illustrated  through  labora- 

tory studies.    These  two  differences  are  (1)  in  the  sexual  organs, 

which  are  many-celled,  and  (2)  in  the  appearance  of  a  new  stage 

in  the  life  cycle  called  the  sporopliyte. 

*  To  THE  INSTRUCTOR  :  The  introduction  to  this  chapter  assumes  that  the 
life  history  of  a  liverwort  or  moss  has  been  studied  in  the  laboratory. 

275 


G<  243.    The  anther- 
idium  of  a  liverwort 


Suie  and  the  mass  of 
^n  cells  within,  in 

which  are  developed 

the  minute  two-ciiiate 


276 


THE  BRYOPHYTES 


283.  The  Sexual  organs.  The  sexual  organs  of  the  bryophytes 
are  many-celled.  They  are  male  and  female  and  each  consists 
of  a  cellular  case,  or  capsule,  in  which 
are  formed  the  respective  gametes,  which 
are  sperms  and  eggs.  It  will  be  remem- 
bered that  the  sexual  organs  of  the  thal- 
lophytes  are,  with  very  few  exceptions, 
one-celled.  The  conspicuous  exceptions 
are  the  plurilocular  sexual  organs  of  the 
lower  brown  algae  (see  Ectocarpus,  Sec. 
235)  and  the  antheridium  of  the  stone- 
worts  (Sec.  230). 

The  sperm-producing  organ,  or  anther- 
idium.    The  antheridium  (Fig.  243)  is  a 
stalked,  oval  or  elliptical  structure,  with 
an   outer    cellular   envelope  inclosing  a 
dense  mass  of  very  small  cubical  cells 
in  which  are  developed  the  sperms.    The 
sperms  are  minute  elongated  or  slightly 
coiled  protoplasts,  with  a  pair  of  cilia  at 
one  end.    The  mature  antheridia  only  dis- 
charge their  sperms  when  wet,  as  after 
heavy  rains  or  dews,  and  the  sperms  then 
FIG.  244.  The  a  r  die  go-   swim  about  in  the  moisture.    At  these 
nium  of  a  liverwort   times  the  plants  are  practically  leading 
an  aquatic  life  like  their  algal  ancestors, 
Thearchegoniainthisgenus       d  ^      development   of  motile  sperms 

hang  down  from  a  special  r 

receptacle  (Fig.  251).  The   in  these  land  plants  shows  clearly  that 
£rj§Ltai±07.£   they  must  have  come  from  forms  with 

archegonium,    while  the    aquatic  life  habits. 

^"cT^l1^        Th*  egg-prodncint,  organ,  or  archego- 
down  into  mucilage  as  the   nium.    The   female   organ  is   called    an 

archegonium  matures.         archegonium.    It  IS  flask-shaped  (Fig.  244), 

and  the  outer  cellular  envelope  incloses  at  maturity  a  row  of  cells. 
The  cell  situated  in  the  enlarged  portion  of  the  archegonium 


THE  SPOROP.HYTE  277 

(venter)  becomes  the  single  egg,  while  the  others  in  the  neck 
region  (Fig.  244,  ri),  called  canal  cells,  break  down  and  their 
substance  becomes  changed  into  mucilage.  The  -archegonia,  like 
the  antheridia,  open  only  when  wet,  the  cells  at  the  tip  separat- 
ing so  as  to  give  a  clear  passage  for  the  entrance  of  the  sperms 
into  the  neck.  The  sperms  are  attracted  to  the  opening  by  cer- 
tain substances  such  as  sugar  contained  in  the  liquefying  muci- 
lage. The  sperms  swim  down  the  neck,  and  one  of  them,  fusing 
with  the  egg,  fertilizes  it.  There  is  much  evidence  that  the 
canal  cells  are  degenerate  gametes,  and  that  the  archegonium 
came  from  a  type  of  sexual  organ  that  originally  produced  a 
number  of  gametes,  as  does  the  antheridium. 

284.  The  sporophyte.  The  term  sporophyte  has  appeared 
before  in  the  accounts  of  the  red  algae  (Sec.  246)  and  sac  fungi 
(Sees.  266,  272)  where  certain  peculiar  fructifications  (cystocarps 
and  ascocarps),  following  the  sexual  process,  alternated  with  the 
sexual  plants.  The  sporophytes  of  the  liverworts  and  mosses 
have  a  similar  position  in  the  life  history,  and  are  likewise 
borne  on  the  parent  plants  and  frequently  called  their  "  fruits." 

The  sporophyte  of  the  liverworts  and  mosses  develops  at 
once  from  the  fertilized  egg,  which  never  becomes  a  resting 
spore  (oospore),  as  in  the  algae.  The  form  is  various  in  differ- 
ent groups.  Most  of  the  mosses  have  long,  stalked  sporophytes 
(Figs.  261,  265),  which  end  in  swollen  spore  cases.  The  liver- 
worts generally  have  much  smaller  sporophytes,  some  of  which 
have  no  stalk  at  all  and  consist  of  the  spore  case  alone.  If  the 
sporophyte  is  small  it  may  remain  inclosed  in  the  base  of  the 
archegonium,  which  becomes  much  enlarged.  But  the  stalked 
sporophytes  either  burst  out  of  the  archegonium,  or  frequently, 
as  in  the  common  mosses,  tear  it  off  and  carry  it  upwards  as  a 
cap-like  structure  (Fig.  265,  B,cal}  called  the  calyptra  (meaning 
a  veil).  The  sporophytes  always  remain  attached  to  the  parent 
plant,  and  finally  develop  spores  asexually  in  the  spore  cases. 
The  spores  are  formed  in  groups  of  four,  called  tetrads,  within 
spore  mother  cells  (Figs.  245,  B,  s;  258,  J5).  These  asexual 


278  THE  BRYOPHYTES 

spores  have  heavy  walls  and  can  survive  the  winter,  frequently 
protected  by  the  spore  case.  It  will  be  remembered  that  in  the 
algse  the  sexually  formed  spore  is  generally  the  protected  resting 
spore. 

285.  Alternation  of  generations.  There  are  thus  two  phases 
in  the  life  history  of  a  liverwort  or  moss.  First,  there  is  the 
plant  which  bears  the  sexual  organs,  and  this  is  called  the 
gametopliyte  (meaning  a  gamete-bearing  plant) ;  second,  there  is 
the  structure  which  arises  from  the  fertilized  egg  and  ends  its 
history  by  developing  asexual  spores,  and  for  this  reason  it  is 
called  a  sporophyte  (meaning  a  spore-bearing  plant).  The  game- 
tophyte  is  developed  from  the  spore,  and  the  sporophyte  from 
the  fertilized  egg.  So  there  is  a  regular  alternation  of  these  two 
phases  in  the  life  history,  the  gametopliyte  producing  sexual 
cells,  or  gametes,  and  the  sporophyte  producing  asexual  spores. 
The  two  phases  are  regarded  as  separate  generations  because 
each  has  its  origin  from  a  distinct  kind  of  reproductive  cell 
(egg  or  spore).  The  gametophyte  is  of  course  a  sexual  genera- 
tion and  the  sporophyte  an  asexual  one.  Their  following  one 
after  the  other  makes  an  alternation  of  generations,—  a  phrase 
which  from  now  on  will  be  frequently  used,  because  it  signifies 
the  most  remarkable  feature  in  the  evolution  of  all  plants  above 
the  thallophytes.  The  simple  sporophytes  of  ancient  bryophytes 
gave  rise  to  the  fern  plants  and  through  them  to  the  large  and 
complicated  seed  plants. 

A  life  history  which  consists  of  an  alternation  of  sporophyte 
and  gametophyte,  as  in  the  liverworts  and  mosses,  may  be 
expressed  by  the  formula 


Gametopliyte  <^  ^  '     ^>  —  Sporophyte  —  asexual  spore 
This  in  an  abbreviated  form  becomes 


—  Gametophyte,  etc- 


&<l>-S-<#-G<*>-S-*p-ef,  etc. 

t/  t/ 


THE   RICCIA  GROUP  279 

One  must  bear  in  mind  these  general  characters  of  the  bryo- 
phytes,  as  the  liverworts  and  mosses  are  separately  taken  up 
and  their  characters  finally  summarized  by  treating  the  sub- 
jects under  the  four  heads  : 

Class  I.     The  liverworts,  or  Hepaticce 
Class  II.  The  mosses,  or  Musci. 
The  origin  and  evolution  of  the  bryophytes. 
Summary  of  the  bryophytes  and  thallophytes. 

CLASS  I.  THE  LIVEKWOKTS,  OE  HEPATIC^ 

286.  The  liverworts.    The  liverworts  grow  most  luxuriantly 
,  in  moist  and  shaded  situations,  some  forms  on  the  ground,  some 

on  rocks,  and  some  on  trees.  There  are  also  certain  aquatic 
liverworts  which  float  on  the  surface  of  the  water,  and  a  few 
very  simple  ones  which  are  entirely  submerged  like  the  algae. 
Thus,  although  most  of  the  types  have  the  land  habit,  some  show 
very  clearly  adaptations  for  the  aquatic  life  of  their  ancestors 
among  the  algae.  The  creeping  habits  of  the  liverworts  probably 
indicate  the  way  in  which  land  plants  arose  and  became  estab- 
lished first  along  the  margins  of  streams,  ponds,  and  marshes 
where  algal  growths  emerged  from  the  water  or  were  left 
stranded  on  the  wet  earth.  These  first  land  liverworts  naturally 
clung  close  to  the  wet  earth  in  the  beginning,  until  the  devel- 
opment of  root-like  systems  of  filaments  (rhizoids),  which  could 
gather  moisture,  permitted  them  to  develop  upright  stems  as 
in  the  mosses.  The  forms  of  the  liverworts  are  various,  as  will 
appear  in  the  following  brief  account  of  the  four  orders. 

287.  The  Riccia  group.    The  simplest  liverworts  (order  Eic- 
ciales)  have  a  flat  plant  body  (gametophyte),  some  forms  float- 
ing on  the  surface  of  the  water  and  others  submerged,  while 
certain  types  grow  close  to  moist  earth.    The  plant  body  is  a 
true  thallus  (Fig.  245,  A),  and  indeed  is  much  simpler  than  the 
plant  bodies  of  many  thallophytes  among  the  brown  and  red 
algae.   The  lower  surface  of  the  thallus  bears  numerous  filaments, 


280 


THE  BRYOPHYTES 


caUed  rhizoids  (from  their  resemblance  to  roots),  and  delicate 
membrane-like  fringes,  which  draw  up  water  from  the  soil  like 

root  hairs  if  the  plant  has 
the  land  habit.  The  thal- 
lus  grows  from  a  number 
of  points  (Fig.  245,  A,  gp) 
situated  in  notches  at  the 
ends  of  lobes  which  fork  in 
pairs  and  finally  split  apart, 
so  that  the  plants  multiply 
very  rapidly  during  the  veg- 
etative season.  The  sexual 
organs  are  borne  on  the  up- 
per surface  of  this  game- 
tophyte,  and  the  sporophyte 
is  a  simple  globular  case 
(Fig.  245,  B),  filled  with 
spores,  which  remains  in- 
closed in  the  base  of  the 
archegonium,  so  that  the 
spores  are  not  set  free  until 
the  decay  of  the  plant.* 

288.  The  Marchantia 
group.    This   large  group 


FIG.  245.   A  floating  liverwort  (Riccio- 
carpus)  and  its  sporophyte 


A,  habit  sketch  of  the  sexual  plant  (game-  (order  Marchantiales)  is 

tophyte)  viewed  from  above,  showing  the  well  represented  bvthe  com- 
position of  the  sporophytes  in  lines  back  / 

of  the  growing  points  gp.  B,  section  of  mon  liverwort  (Marchantia 

a  sporophyte  contained  within  the  parent  p0lym0rplia\  which  grows 

archegonium,  whose  neck  n  is  still  pres-  ±                                 ,    .               . 

ent:   a,  spores  in  groups  of  four  (tetrads)  Oil   the  ground    in    moist 


within  the  spore  mother  cells;  w,  remains 
of  the  wall  of  the  sporophyte 


situations.    The  ribbon-like 
thallus  of  species  of  Mar- 
chantia (Fig.  246)  forks  regularly,  but  one  of  the  branches  is 


*To  THE  INSTRUCTOR  :  These  points  are  admirably  illustrated  in  the  large 
floating  form  (Ricciocarpusnatans),  which  is  not  uncommon  and  is  an  excellent 
type  for  laboratory  study,  although  Marchantia  is  the  form  most  generally  used. 


THE  MARCHANTIA  GROUP 


281 


almost  always  larger  and  stronger  than 
the  other.    The  lower  surface  bears  nu- 
merous filaments  and  fringes  which  are 
formed  in  front  of  the  growing  points, 
protecting  them,  and  later 
become  distributed  along 
the  lower  surface.    The 
upper  surface  is  marked 
by  diamond-shaped  areas 
(Fig.  247,  A)  which  show 
the  position  of  curious 
large   air   chambers  (Fig. 


Fi<;.  24(>.   A  Marchantia  form  (Marchantia 
disjuncta) 


247   13}  that  contain  very    ./>,  female  receptacle ;  c,  cups  producing  buds. 

—  After  Sullivant 

numerous  filaments  whose 

cells  have  well-developed  chloroplasts.    These  filaments  perform 


FIG.  247.    Structure  of  the  thallus  of  Marchantia 

A,  surface  of  thallus,  the  diamond-shaped  areas  marking  air  chambers ;  7>,  a  section 
through  the  middle  region  of  the  thallus  showing  air  chambers  above,  filled  with 
branching  green  filaments,  and  the  fringes  and  root-like  hairs  (rhizoids)  on  the 
lower  surface ;  C,  surface  view  of  the  pore  which  opens  into  an  air  chamber 


282 


THE  RRYOPHYTES 


the  greater  part  of  the  chlo- 
rophyll work  (photosynthesis) 
of  the  plant,  and  the  chambers 
are  developed  as  protective 
structures  around  them.  Each 
chamber  is  open  above  to  the 
air  by  a  circular  pore  (Fig. 
247,  (7),  which  can  be  easily 
seen  in  the  center  of  each 
diamond-shaped  area.  This 
specialization  of  the  upper  sur- 

F.o.  248.  The  cups  and  buds  of          faCe  °f  tlle  *"««*«»««  P^nt 

Marchantia  to  a  light  relation  gives  it  a 

A,  cup-bearing  plant;  B,  section  of  a  cup   general   resemblance   to   the 
showing  the  buds  arising  from  its  bot-   cell  structure  of  leaves  in  seed 

torn  ;  C,  a  bud  showing  the  two  growing       -.  , 

points;  D,  young  plant  developing  from    plants  and  terns. 

thebud  Some  individuals  of  Mar- 

chantia (Fig.  248,  A)  will  usually  be  found  bearing  cups  (cupules) 
which  contain  numerous  green  bodies.  These  are  many-celled 
reproductive  organs,  called 
buds  (gemmse),  which  de- 
velop from  the  bottom  of  the 
cup  (Fig.  248,  B).  Each  bud 
has  two  notches  at  opposite 
sides  (Fig.  248,  C),  which 
become  two  growing  points 
when  the  structure  falls  on 
its  side  upon  damp  earth  and 
begins  to  grow  (Fig.  248,  D). 
This  is  a  characteristic  and 
very  successful  method  of 
rapid  asexual  multiplica- 


tion in  Marchantia. 

The  sexual  organs  of 
Marchantia  are  developed 


FIG.  249.   The  male  plant  of  Marchantia 

A,  male  plant  bearing  antheridial  receptacles; 
B,  lengthwise  section  of  a  receptacle  (semi- 
diagrammatic),  showing  a  row  of  sunken 
antheridia  upon  the  upper  surface;  the 
youngest  lie  just  back  of  the  notches  in  the 
receptacle,  which  are  the  growing  points; 
air  chambers  are  also  shown  on  the  upper 
surface 


THE  MARCHANTIA  GROUP 


283 


FIG.  250.   Female  plant  of 
Marchantia 

Showing  the  umbrella-like  arche- 
gonial  receptacles  in  various 
stages  of  development 

on  stalked,  umbrella-like 
receptacles,  which  are 
really  much-modified 
branches  of  the  thallus. 
They  bear  either  anther- 
idia  or  archegonia,  and 
the  two  sexual  organs  are 
not  found  together  on  the 
same  plant.  The  anther- 
idial  receptacle  (Fig.  249, 
A)  is  shorter  than  the 
archegoiiial,  and  the  top 
is  a  flattened  disk  with  a 
lobed  or  scalloped  margin. 
The  antheridia  (Figs.  243, 
249,  B}  lie  in  cavities  or 
pits  along  radiating  lines 


FIG.  251.   The  female  receptacle  of 
Marchantia 

A,  portion  of  a  lengthwise  section  of  a  young 
receptacle  (semi-diagrammatic),  showing  a 
row  of  archegonia  hanging  down  from  the 
lower  surface,  the  youngest  being  nearest  the 
stalk :  air  chambers  are  present  on  the  upper 
surface ;  I,  one  of  the  finger-like  lobes  back  of 
the  section,  the  diamond-shaped  areas  indi- 
cating air  chambers.  B,  a  young  sporophyte 
within  the  parent  archegonium:  the  region 
which  is  to  become  the  spore  case  is  indicated 
by  the  cross  lines,  and  the  small  foot  is  at- 
tached to  the  base  of  the  archegonium;  e,  a 
special  envelope  developed  around  the  arche- 
gonia of  Marchantia 


284 


THE  BRYOPHYTES 


on  the  upper  surface  of  the  receptacle,  and  the  youngest  anther- 
idia  are  found  nearest  the  notches  which  mark  the  position  of 
the  growing  points  along  the  edge  of  the  disk.  The  archegonial 
receptacle  (Fig.  250)  is  larger  than  the  antheridial,  and  the  top 
is  bent  back  into  several  long,  finger-like  projections  like  the 
ribs  of  an  umbrella.  Numerous  archegonia  are  formed  in  lines 
(Fig.  251,  A)  on  the  under  side  between  the  lobes,  and  are  pro- 
tected by  singular  fringes.  The  youngest  archegonia  are  formed 


FIG.  252.   Sporophytes  and  receptacles  of  Marchantia 

A,  lower  view  of  an  old  female  receptacle,  showing  the  sporophytes  in  rows  between 
the  fringes  /,  like  peas  in  a  pod.  B,  section  of  a  receptacle  (diagrammatic), 
showing  a  mature  sporophyte  anchored  by  its  foot  and  projecting  beyond  the 
fringe  /:  the  spore  case  is  open,  exposing  the  mass  of  elaters  el;  a  young 
sporophyte  is  shown  at  the  right  still  inclosed  within  the  archegonium  (ca- 
lyptra) ;  e,  special  envelopes  around  the  archegonia  and  sporophytes 

nearest  the  stalk,  so  that  the  older  ones  lie  farther  out,  —  an 
arrangement  exactly  opposite  from  that  of  the  antheridia.  This 
is  explained  by  the  fact  that  the  growing  points  which  lie  be- 
tween the  lobes  grow  downward  and  underneath  towards  the 
stalk.  The  edge  of  the  disk  is  thus  bent  back  on  itself,  and  the 
lower  surface  is  really  an  extension  of  the  upper  surface. 

A  number  of  archegonia  may  be  fertilized  in  Marchantia,  and 
their  eggs  then  develop  sporophytes  in  radiating  rows  on  the 


THE  MARCHANTIA  GROUP 


285 


lower  surface  of  the  receptacle  (Fig.  252,  A)  between  the  fringes. 
The  sporophyte  is  more  complex  than  in  the  Riccia  types.  The 
lower  part  (Figs.  252,  B ;  253,  A)  becomes  a  small  organ  of  at- 
tachment to  the  gametophyte,  called  the  foot,  through  which 
it  obtains  water  with  food  in  solution.  The  upper  part  becomes  a 
spore  case,  developing  numerous  spores,  and  among  them  spirally 
marked  filaments,  termed  elaters  (Fig. 
253,  B),  which  are  stiff  and  elastic  and 
help  to  distribute  the  spores.  The  elaters 
are  developed  from  cells  in  the  young 
spore  case.  The  spore  case  is  carried 
beyond  the  fringe  of  the  receptacle  (Fig. 
252,  B)  by  the  elongation  of  the  region 
above  the  foot,  which  forms  a  stalk.  The 
presence  of  a  foot  and  stalk  in  addition 
to  the  spore  case  marks  a  decided  ad- 
vance over  the  simple  sporophytes  of  the 
Riccia  types,  which  consist  of  the  spore 
case  alone. 

It  is  very  important  to  note  that  the 
sporophyte  has  this  close  attachment  to 
the  gametophyte  and  is  dependent  upon 
it  for  water  and  food  in  solution,  because 
it  shows  that  the  sporophyte  of  the  liver- 
worts really  lives  in  large  part  like  a 
parasite  upon  the  gametophyte  as  a  host. 

289.  The  Jungermannia  group.  This 
assemblage  (order  Jungermanniales)  is 
very  much  the  largest  group  of  the  liverworts  and  contains 
more  than  three  thousand  species.  They  are  known  as  the  leafy 
liverworts  because  most  of  them  have  long  stems,  with  delicate, 
moss-like  leaves.  The  leafy  liverworts  are  frequently  mistaken 
for  mosses,  since  they  are  common  on  tree  trunks  and  in  shaded 
situations.  But  they  have  a  creeping  habit,  and  there  are  two 
crowded  rows  of  large  leaves  (Fig.  254,  A),  one  on  each  side  of 


FIG.  253.   The  sporophyte 
of  Marchantia 

A,  longitudinal  section  of 
sporophyte  showing  spore 
ease  and  foot  attached  to 
the  base  of  the  archego- 
nium:  e,  a  special  envel- 
ope. It,  an  elater 


FIG.  254.   The  female  plant  of  a  leafy  liverwort  (Porella) 

A,  habit  sketch  of  the  upper  surface,  with  the  two  rows  of  leaves  at  the  sides ;  B, 
a  portion  of  lower  surface,  showing  the  third  row  of  small  leaves  (amphigas- 
tria) ;  O,  the  stalked  sporophytes  with  open  spore  cases  sc ;  D,  a  sporophyte 
with  the  spore  case  split  lengthwise  into  four  parts.  —  After  Campbell 


FIG.  255.   The  antheridia  of  a  leafy  liverwort  (Porella) 

A, portion  of  male  plant,  illustrating  the  small  antheridial  branches  at  the  side; 
-B,  section  of  an  antheridial  branch,  showing  two  antheridia  situated  ju£t  above 
the  attachment  of  the  leaves  I ;  C,  the  much-elongated  sperm,  with  the  two  cilia 
at  one  end  and  the  remains  of  the  parent  cell  at  the  other.  —  C,  after  Campbell 

286 


THE  JUNGERMANNIA  GROUP 


287 


the  stem,  and  a  third  row  of  small  modified  leaves  ou  the  lower 
surface  (Fig.  254,  B}.  The  stems  of  mosses,  on  the  contrary,  are 
almost  always  upright,  and  the  leaves  are  arranged  radially,  so 
that  the  stem  has  no  upper 
or  lower  surface. 

The  antheridia  of  the 
leafy  liverworts  are  borne 
singly  along  the  stem  at 
the  bases  of  the  lateral 
leaves  (Fig.  255,  B)  on  cer- 
tain branches  which  are 
frequently  much  smaller 
than  the  vegetative  shoots 
(Fig.  255,  A).  The  arche- 
gonia  are  developed  in  clus- 
ters at  the  ends  of  branches. 

The  sporophyte  (Fig. 
256,  A)  has  a  stalk  which 
elongates  rapidly  just  be- 
fore the  spores  are  ready  to 
be  shed,  so  that  the  spore 


case    is    raised  above  the 


FIG.  256.   The  sporophyte  of  a  leafy 
liverwort  (Porella) 


in  the  parent  archegonium,  whose  neck  n  is 
shown  above,  the  foot  deeply  sunken  in  the 
tissue  of  the  gametophyte :  a,  archegonia  of 
the  terminal  group,  which  were  not  ferti- 
lized ;  I,  leaf-like  envelopes.  B,  the  four-lobed 
spore  mother  cell,  which  develops  four  spores 
(tetrad).  C,  an  elater 


gametophyte  (Fig.  254,  C),    A  section  of  a  sporophyte  still  contained  with- 

as  in  the  mosses.  How- 
ever, the  spore  case  is  much 
less  complex  than  that  of 
the  mosses,  being  a  simple 
capsule  that  splits  length- 
wise into  four  parts  at  maturity  (Fig.  254,  D).  There  are  spirally 
thickened  filaments,  or  elaters  (Fig.  256,  C),  among  the  spores, 
as  in  Marchantia,  and  these  structures  are  not  found  in  the 
mosses.  The  foot  of  the  sporophyte  (Fig.  256,  A)  is  always  well 
developed  in  the  leafy  liverworts.* 

*  To  THE  INSTRUCTOR  :  Good  material  of  the  leafy  liverworts  frequently 
furnishes  better  subjects  for  type  study  of  the  liverworts  than  Marchantia. 


288 


THE  BRYOPHYTES 


290.  The  Anthoceros  group.  These  types  (order  Anthocero- 
tales)  are  considered  the  highest  of  the  liverworts  because  of 
their  more  complicated  sporophytes.  The  gametophytes  are  thal- 
loid  (Fig.  257),  somewhat  irregular  in  outline,  and  more  simple 
in  structure  than  those  of  Marchantia.  The  sporophytes  are  an 
inch  or  more  in  height,  and  grow  up  from  the  gametophyte  like 
blades  of  grass.  The  upper  portion  splits  lengthwise  into  halves 
at  maturity. 

The  spores  of  Anthoceros  do  not  all  mature  at  once,  as  in  other 
liverworts,  but  new  spores  are  formed  at  the  base  of  the  sporo- 
phyte as  the  older  mature  (Fig.  258,  A)y. 
and  there  is  a  continuous  elongation  of 
the  structure  during  the  summer  from  a 
basal  region  of  growth.  The  cells  com- 
posing the  wall  of  this  long  sporophyte 
contain  large  single  chromatophores  (Fig. 
258,  E),  and  there  are  breathing  pores, 
or  stomata  (singular,  stoma,  meaning  a 
mouth),  on  the  surface  (Fig.  258,  D),  which 
lead  into  intercellular  spaces  in  the  green 
tissue  beneath.  Consequently  the  sporo- 
phyte is  able  to  manufacture  its  own  food 
The  thaiioid  sexual  plants  ty  photosynthesis,  as  any  green  plant  may 

(gametophytes),  with  the     /  r  J 

sporophytes  s  in  various   do.   But  it  depends  upon  the  gametophyte 
stages  of  development       f  or  its  SUppiy  Of  Water,  which  is  absorbed 

through  a  large  bulbous  foot  (Fig.  258,^1)  that  is  deeply  im- 
bedded in  the  thallus  of  the  gametophyte. 

If  the  base  of  this  sporophyte  could  establish  a  root-like 
structure  growing  in  the  soil,  it  might  live  independently  of  the 
parent  gametophyte,  for  it  has  chlorophyll-bearing  tissues  in 
communication  with  the  air  through  stomata,  just  as  in  the 
ferns  and  seed  plants.  And  it  has  also  the  power  of  indefi- 
nite growth  from  its  basal  region  (Fig.  258,  A),  limited  only  by 
the  length  of  the  summer  season.  These  peculiarities  of  the 
sporophyte  of  Anthoceros  are  very  suggestive  of  the  way  in 


FIG.  257.   Habit  sketch 
of  Anthoceros 


THE  ANTHOCEROS  GROUP 


which  higher  plants  must  have  arisen 
from  forms  somewhat  like  the  liver- 
worts, a  subject  which  we  shall  con- 
sider later  in  our  account  of  the  ferns 
(Sec.  331).  Of  all  the  bryophytes, 
this  seems  to  be  the  genus  which 
most  closely  approaches  the  higher 
plants.  This  account  of  plant  evo- 
lution is  now  well  started  towards 
the  higher  conditions  of  plant  devel- 
opment, namely,  those  of  the  ferns 
and  seed  plants  whose  sporophyte 
generations  are  independent  plants 
with  roots,  stems,  and  leaves,  and 
which  comprise  the  most  independ- 
ent and  successful  vegetation  on  the 
earth. 

CLASS  II.   THE  MOSSES,  OR 
MUSCI 

291.  The  mosses.  The  mosses  are 
very  much  more  numerous  than 
the  liverworts.  Some  of  the  com- 
mon kinds  grow  in  extensive  carpets 
on  hillsides  and  in  forests,  becom- 
ing important  factors  in  the  plant 

A,  longitudinal  section  (semi-diagrammatic) 
through  the  base  of  the  sporophyte,  showing 
the  large  foot  imbedded  in  the  tissue  of  the 
gametophyte,  the  region  of  growth,  and  the 
spore-producing  tissue  which  forms  a  cylin- 
der in  the  center  of  the  stalk ;  B,  a  group 
(tetrad)  of  four  spores  (three  shown)  in  a 
spore  mofher  cell;  C,  spores;  D,  a  stoma 
viewed  from  the  surface ;  E,  section  through 
a  stoma,  showing  cells  with  large  single 
chromatophores  under  the  surface  layer 
(epidermis) 


289 


FIG.  258.   The  sporophyte  of 
Anthoceros 


290 


THE   BRYOPHYTES 


formations  of  many  regions.    The  peat  mosses  are  the  chief  in- 
habitants of  certain  kinds  of  bogs  and  pond  margins.    The  mosses 

therefore    constitute   a  group   of 
considerable   importance    in   the 
plant  population  of  the  earth,  while 
the  liverworts  are  for  the  most 
part    confined  to   rather    special 
life  habits  and,  with  the  exception  of  the 
leafy  liverworts,  are  not  rich  in  species  or 
numerous  in  individuals.    Almost  all  of  the 
mosses  fall  into  two   groups,  which  may  be 
called  the  peat  mosses  and  the  common  mosses. 

292.  The  peat  mosses.   The  peat  mosses  (order 
Spliagnales)  are  very  remarkable  for  their  struc- 
ture and  life  habits.    There  is  only  a  single  genus, 
Sphagnum,  with  about  two  hundred  and  fifty 
species.    The   plants   (gametophytes)   have  long 
stems,  with   delicate,  leafy  branches,   some    of 
which  grow  downward  and  soak  up  water,  while 
the  rest  form  a  dense  cluster  at  the  top  (Fig.  259). 
The  peculiar  structure  of  these  mosses  allows 
them  to  absorb  and  hold  water  like  a  sponge,  for 
which  reason  they  are   used  by  gardeners  for 
packing  around  plants  and  flowers.    The  dried 
moss  is  sometimes  used  for  bedding  in  stables. 
The  sexual  organs  (antheridia  and  archegonia) 
are  formed  very  early  in  the  spring  or  in  the 
late   winter,    and    the   fertilization    of    the    egg 
FIG.  259        leads  at  once  to  the  development  of  a  sporophyte. 
The  peat  moss    The  sporophytes  are  large,  smooth  capsules  (Fig. 
(Sphagnum)     260,  J),  which  appear  to  have  stalks,  but  these 
are  really  special  developments  of  the  gametophytes.    The  spore 
case  is  attached  to  the  top  of  the  stalk  by  a  large  foot  and  opens 
by  a  cover  (Fig.  260,  B,  c),  which  falls  off.    The  spores  on  ger- 
mination produce  small  flat  cell  plates,  out  of  which  the  leafy 


THE    PEAT   MOSSES 


291 


peat  mosses  arise  from  special  buds.  These  cell  plates  suggest 
the  simple  thalloid  gametophytes  of  the  liverworts,  and  the  leafy 
structure  is  perhaps  a  special  development  from  them. 

The  peat  mosses  live  in  bogs  and  swamps  and  are  especially 
common  in  northerly  regions  and  in  the  mountains,  where  they 


FIG.  260.  The  sporophyte  of  the  peat  moss  (Sphagnum) 

A,  group  of  the  sporophytes  on  stalks,  which  are  really  growths  from  the  game- 
tophyte.  n,  longitudinal  section  through  a  sporophyte,  showing  the  large  foot 
imbedded  in  the  top  of  the  stalk:  o,  the  remains  of  the  parent  archegonium, 
with  the  neck  still  present ;  s,  spore  chamber ;  c,  cover 

grow  over  wet  rocks,  sometimes  covering  large  areas.  They 
develop  so  rapidly  that  they  frequently  fill  ponds  and  bogs. 
The  first  growth  is  around  the  edges  of  the  pond,  but  this  grad- 
ually works  inwa,rcf,  until  finally  the  whole  surface  is  covered 
with  peat  moss.  Such  conditions  produce  quaking  bogs,  for  the 
surface  is  not  firm  enough  to  hold  any  large  animal  which 
might  walk  upon  it.  Quaking  bogs  become  firmer  as  the  lower 


292  THE  BRYOPHYTES 

parts  of  the  peat  mosses  die  and  form  a  fibrous  deposit  below. 
These  deposits  may  grow  to  be  many  feet  in  thickness,  and 
finally  become  so  firm  that  they  can  be  cut  out  in  blocks.  Such 
blocks  when  dried  are  used  for  fuel,  especially  in  Ireland  and 
in  the  Highlands  of  Scotland.  There  are  regions  of  the  north- 
ern United  States,  Canada,  Europe,  and  Asia  where  the  peat 
mosses  cover  immense  territories,  and  there  are  innumerable 
bogs  filled  with  deposits  of  peat  which  may  sometime  become 
important  sources  of  fuel  supply. 

Peat  bogs  are  generally  poorly  drained  or  not  drained  at  all, 
and  the  water  becomes  very  rich  in  certain  organic  acids  that 
result  from  the  partial  decomposition  of  the  vegetation.  The 
accumulation  of  these  acids  renders  the  water  unfit  for  the 
growth  of  bacteria  and  is  largely  responsible  for  the  preservation 
from  decay  not  only  of  the  remains  of  the  peat  mosses  but  of 
other  plants  with  them.  It  is  said  that  whalers  and  other  ships 
from  the  New  England  coast  when  starting,  on  long  voyages 
preferred  to  take  their  supplies  of  drinking  water  from  peat 
bogs  because  of  its  keeping  qualities.  Occasionally  the  bones 
of  extinct  animals,  such  as  the  mammoth  and  mastodon,  are 
found  in  peat,  since  these  gigantic  creatures  became  mired  in 
the  soft  bogs  of  former  periods. 

As  a  quaking  bog  becomes  firmer,  other  plants  begin  to  grow 
among  the  peat  mosses.  Certain  grasses  appear,  some  charac- 
teristic orchids  (Calopogon,  Pogonia,  Arethusa,  Cypripedium, 
etc.),  the  insectivorous  plants  Sarracenia  (Fig.  311)  and  Drosera 
(Fig.  312),  such  heaths  as  the  swamp  cranberry,  swamp  blue- 
berry, swamp  azalea,  and  Labrador  tea,  and  certain  trees,  as  the 
larch  or  tamarack  (Larix),  black  spruce  (Picea),  the  arbor  vita? 
(Thuya),  and  the  white  cedar  (Chamcecyparis).  These  plants,  in 
various  combinations  with  the  peat  mosses,  form  very  character- 
istic associations,  and  they  furnish  some  of  the  best  illustrations 
of  what  the  ecologist  calls  plant  formations.  The  northeastern 
United  States  and  Canada  are  full  of  examples  of  this  interest- 
ing feature  in  the  natural  history  of  the  Sphagnum  swamp. 


THE   COMMON  MOSSES 


293 


293.  Common  mosses.* 

The   common   mosses   (or- 
der Bryales)   are    familiar 
because  of  the  occurrence 
of  numerous  species  with 
conspicuous  upright  stems, 
which    develop  the    long- 
stalked   sporophytes    with 
characteristic   terminal 
spore  cases  (Figs.  261,265). 
It  is  a  very  large  group, 
containing  over  eight  thou- 
sand species,  and  is  by  far 
the  most  numerous  assem- 
blage  in   the    bryophytes. 
These  mosses  grow 
in  the  greatest  va- 
riety of  situations, 
—  in  swamps  and 
bogs,  in  the  water 
of  streams,  in  moist 
and  shaded  woods, 
in  open  fields,  and 
on   relatively   dry 
hillsides  and  rocks. 
They   perform   an 
important   service 
to  plant   life    in 
holding  back  much 
of   the   rainfall, 
allowing  it   to 
sink  into  the  earth 


FIG.  261.   A  common  moss  (Catharinea  undulata) 


Showing  the  branching  leafy  moss  plants  (gameto- 
phytes)  attached  to  the  root-like  mass  of  protonemal 
filaments  and  bearing  sporophytes.  —  After  Sachs 


*  To  THE  INSTRUCTOR  :  In  a  short  course  it  is  best  to  present  the  life  his- 
tory of  bryophytes  through  a  somewhat  detailed  study  of  one  of  the  common 
mosses,  followed  by  general  studies  of  a  variety  of  forms  of  mosses  and  liver- 
worts. 


294  THE  BRYOPHYTES 

instead  of  running  rapidly  off  in  floods.  The  lichens  and  mosses 
are  among  the  first  plants  to  appear  on  barren  soil  or  exposed 
rocks  and  cliffs,  and  are  also  the  plant  pioneers  that  push 
their  way  up  mountains  and  into  the  arctic  regions  where  no 
other  vegetation  can  live. 

294.  The  life  history  of  a  moss.  The  life  history  of  the 
common  mosses  is  more  complex  than  that  of  a  liverwort.  The 
moss  spore  does  not  develop  directly  into  the  leafy  moss  plant.  It 


prim 


FIG.  262.   The  protonema  of  a  common  moss  (Funaria) 

prim,  primary  shoot;    br,  branches  from  primary  shoot;  pi,  young  moss  plant 
or  bud.  —  After  Sachs 

produces  a  preliminary  filamentous  growth,  called  the  protonema 
(meaning  preliminary  thread),  which  sometimes  forms  an  exten- 
sive network  over  the  ground,  resembling  at  first  sight  such 
terrestrial  algse  as  certain  species  of  Vaucheria.  The  proton emal 
filaments  (Fig.  262),  however,  consist  of  cells  placed  end  to  end 
(they  are  never  coenocytic) ;  they  have  generally  oblique  cross 
walls  and  contain  numerous  disk-shaped  chloroplasts.  There 
are  no  algae  known  which  the  protonema  resembles  in  detail, 
and  yet  this  phase  in  the  life  history  suggests  what  may  have 
been  the  life  habits  of  ancestors  of  the  mosses.  Certain  cells  of 


THE  LIFE  HISTORY  OF  A  MOSS 


295 


the  protonema  change  their  methods  of  cell  division  and  develop 
small  buds  (Fig.  262,  pi)  which  grow  into  the  leafy  moss  plants 
(F"ig.  263).  One  moss  spore  may  give  rise  to  a  great  quantity  of 
protonema,  which  by  means  of  the  numerous  buds  will  form  a 
large  group  or  even  a  turf  of  moss  plants.  Therefore  the  protonema 
is  a  very  effective  means  of  establishing  the  large  carpets  of  moss 
vegetation.  The  leafy  moss  plant  develops  the  sexual  organs  in 
clusters  at  the  top  of  the  stem 
and  has  further  peculiarities  of 
structure  which  will  be  described 
later.  The  protonema  together 
with  the  leafy  moss  plant  consti- 
tute the  sexual  or  gametophyte 
phase  of  the  life  history. 

The  fertilization  of  an  egg  in 
an  arehegonium  starts  at  once  the 
development  of  the  sporophyte, 
often  called  the  moss  fruit.  The 
fertilized  egg  gives  rise  to  a  many- 
celled  structure  (Fig.  264,  A), 
which  establishes  a  growing  point 
above  and  a  foot  attachment  to 
the  gametophyte  below.  This 
young  sporophyte  is  contained  at  FIG  20g  A  young  plant  of  a  com. 
first  entirely  within  the  parent  mon  moss  (Webera) 

arehegonium,  which  enlarges  with   Showing  its  attachment  to  the  proto- 

its  development  (Fig.  264,  B,  a).  nemal  filaments  which  bear  repro- 
T^  ,  n  v,  ,,  ductive  buds  6 

But   finally    the   growth   of  the 

sporophyte  is  so  rapid  that  the  arehegonium  is  torn  away  at  its 
base  and  borne  upwards  on  the  elongating  stalk  of  the  sporo- 
phyte. The  remnant  of  the  arehegonium  then  covers  the  tip  of 
the  stalk  like  a  cap  (Fig.  265,  B,  cal)  and  is  called  the  calyptra 
(meaning  a  veil),  which  must  serve  a  useful  purpose,  protecting 
the  delicate  growing  tip  of  the  sporophyte.  Finally,  the  tip  of 
the  sporophyte  enlarges  and  becomes  the  complex  spore  case 


FIG.  264.  Developing  sporophytes  of  a  com- 
mon moss  (Funaria) 

A,  very  young  stage,  showing  the  early  cell 
divisions  of  the  egg ;  B,  older  sporophyte  just 
before  the  archegonium  a  is  torn  away  from 
the  gametophyte  and  carried  upward  as  the 
calyptra.  The  base  of  the  sporophyte  has 
now  grown  down  into  the  tip  of  the  leafy  moss 
plant  (gametophyte)  and  is  firmly  anchored 
to  it.  —  After  Sachs 

296 


B 


FIG.  265.   A  common  moss 
(Polytrichum  commune) 

A,  male  plant,  showing  cup-like 
tip  containing  the  antheridia. 
JB,  female  plant  with  the  spo- 
rophyte :  col,  cap,  or  calyptra, 
over  the  developing  spore  case. 
C,  a  mature  spore  case  with 
the  calyptra  removed 


THE  LEAFY  MOSS  PLANT  297 

(Fig.  265,  C).  The  development  of  the  asexual  spores  in  the 
spore  case  ends  the  life  history  of  the  moss  plant,  which  may 
be  formulated  as  follows  : 


t    /  protonema  and  \  ^sperm^        „ 
Gametophyte  (£*    mnsa    ,.    <    Mlfl   >  -  Sporophyte 


Jeafy  moss  planty     -    egg 

—  asexual  spore  —  Gametophyte,  etc. 

This  in  abbreviated  form  becomes 

G<1>  -S~sp-  £<*>  -  S-sp-G,  etc. 

This  formula  is  identical  with  the  general  life-history  formula 
presented  for  the  bryophytes  in  Sec.  285,  and  it  is  clear  that 
gametophyte  and  sporophyte  alternate  with  one  another. 

295.  The  leafy  moss  plant.  The  leafy  moss  plant  is,  of 
course,  the  conspicuous  part  of  the  gametophyte  phase  of  the 
life  history.  It  consists  of  an  upright  stem,  branching  in  some 
forms,  with  the  leaves  almost  always  distributed  spirally.  The 
symmetry  of  the  plant  is  therefore  radial  instead  of  having  an 
upper  and  a  lower  side  (dorsiventral)  as  in  the  leafy  liverworts. 
The  leaves  consist  for  the  most  part  of  simple  plates  of  cells, 
which  in  some  forms  can  become  dry  and  still  retain  their 
vitality,  freshening  up  with  the  next  rain.1  The  moss  plant  is 
fastened  to  the  earth  by  filaments  of  protonema  (Fig.  263), 
which  grow  out  from  the  base  of  the  stem  and  form  a  dense 
network  underneath  the  moss  plants  (Fig.  261).  This  proto- 
nema becomes  brown  with  age  and  serves  as  a  system  of  root-like 
filaments,  or  rhizoids,  by  which  the  moss  plant  obtains  water 
from  the  soil.  The  growth  of  the  stem  normally  ends  with  the 
production  of  a  terminal  group  of  sexual  organs,  both  of  which 
(antheridia  and  archegonia)  are  found  on  the  same  plant  in 
some  species  and  on  different  plants  (male  and  female)  in 
others.  Male  plants  are  generally  smaller  than  the  female  ones 

1  The  cells  of  the  moss  leaf  are  excellent  subjects  for  study  and  have 
been  described  in  Sec.  195  and  illustrated  in  Fig.  169. 


298 


THE   BRYOPHYTES 


and  more  easily  distinguished  (Fig.  265,  A)  because  the  orange 
or  reddish-brown  clusters  of  antheridia  lie  exposed  at  the  tip 
of  the  stem  and  are  sometimes  surrounded  by  a  circle  or 
rosette  of  modified  or  colored  leaves.  Female  plants  are  larger, 
and  the  archegonia  are  hidden  by 
enveloping  leaves,  which  must  be 
picked  off  to  expose  these  sexual 
organs.  The  antheridia  (Fig.  2  6  6,  a) 
and  archegonia  (Fig.  268,  A)  are 

^^;&.tovr>X~y* 

C 


FIG.  266.   Section  through  the  tip  of  the 
male  plant  of  a  moss  (Funaria) 

a,  antheridium ;  /,  sterile  filament,  or  para- 
physis ;  I,  leaf 


FIG.  267.  The  antheridium  of 
a  common  moss  (Funaria) 

a,  antheridium;  h,  escaping 
sperms ;  c,  a  single  sperm  in 
its  parent  cell.  —  After  Sachs 


sometimes  numerous  in  the   clusters   and  lie  among  hair-like 
structures   (paraphyses). 

The  mature  antheridia  and  archegonia  open  only  when  wet 
by  the  swelling  and  separation  of  a  group  of  cells  at  their  tips. 
The  sperms  (Fig.  267,  b)  are  discharged,  then,  after  rains  or  heavy 
dews,  so  that  the  moss  at  that  time  is  practically  living  an 


THE  SPOROPHYTE  OF   THE  MOSS 


299 


I  7 


aquatic  life.  The  archegonia  (Fig.  268,  B)  have  very  long  necks, 
and  the  relatively  small  egg  lies  at  the  bottom  as  in  a  flask. 
The  sperms  are  attracted  to  the 
mouth  of  the  open  archegonium  by 
substances  in  the  mucilage  within 
the  neck,  one  of  which  at  least  is 
sugar.  They  swim  down  the  neck 
to  the  egg,  and  one  of  them  fertil- 
izes it. 

296.  The  sporophyte  of  the 
moss.  The  sporophytes  of  some  of 
the  common  mosses  are  the  most 
complex  found  among  the  bryo- 
phytes,  with  the  possible  exception 
of  those  of  Anthoceros.  There  is 
generally  a  long  stalk  which  bears 
a  large  spore  case  (Fig.  269,  A).  The 
structure  of  the  spore  case  is  very 
elaborate.  A  cover  (operculum)  is 
formed  at  the  end,  which  falls  off 
so  that  the  spores  may  escape  from 
within.  In  many  mosses  the  cover 
is  loosened  and  thrown  off  by  an 
interesting  mechanism,  which  is 
sometimes  very  highly  developed. 

There  may  be  a  circle  of  cells  with  FlG-  268'  Section  through  the 
,,  .   ,  .  .._    ,        tip  of  a  female  plant  of  a  moss 

thickened  and  otherwise  modified      (Funaria) 


cell  walls,  forming  a  well-defined 
ring  (Fig.  269,  A,  r)  around  the  spore 
case  underneath  the  cover.  These 
cells  change  their  form  when  wret, 
sometimes  swelling  greatly  (Fig. 
269,  (7), and  thus  loosen  or  tear  the  cover  away  from  the  spore  case. 
The  rim  of  the  opening  formed  when  the  cover  falls  off  is 
surrounded  by  a  circle  of  pointed  triangular  structures  called 


A,  group  of  archegonia  a:  £,leaf.  B, 
an  archegonium  in  detail,  show- 
ing enlarged  basal  portion  e  with 
the  egg,  and  the  neck  n  above  with 
its  row  of  canal  cells:  m,  mouth. 
—  After  Sachs 


300 


THE  BRYOPHYTES 


teeth  (Fig.  269,  B,  t)t  which  meet  at  the  center  of  the  opening 
when  folded  inwards.  The  number  of  teeth  is  fixed  for  differ- 
ent mosses.  Under  the  circle  of  teeth  various  mosses  have 
another  circle  of  much  more  delicate  segments  (Fig.  269,  J5,  s) 


FIG.  269.   The  spore  case  of  a  common  moss  (Bryum) 

A,  the  closed  spore  case :  c,  the  cover  (operculum) ;  r,  the  ring.  B,  the  rim  of 
an  open  spore  case,  showing  the  outer  circle  of  teeth  t,  inside  of  Which  is  indi- 
cated the  inner  circle  of  delicate  segments  s:  sp,  spores.  C,  the  cover  after 
remaining  for  a  minute  in  water:  the  cells  of  the  ring  r  have  absorbed  the 
water  and  have  swollen  so  that  the  ring  has  broken  aud  curled  backwards  on 
two  sides 

of  the  same  number  and  general  form.  The  teeth  are  sensitive 
to  moisture,  curling  inwards  and  outwards  with  changes  in  the 
amount  of  vapor  in  the  air,  and  by  these  movements  they  prob- 
ably help  in  some  types  to  empty  the  case  of  its  spores,  retain- 
ing them  in  wet  weather  and  letting  them  fall  out  in  dry. 


THE  SPOROPIIYTE  OF  THE  MOSS 


301 


The  lower  portion  of  the  spore  case  has  stomata  (Fig.  270,  D), 
and  there  is  much  chlorophyll-bearing  tissue  in  the  moss  fruit 
that  is  capable  of  doing  the  work  of  photosynthesis  just  as  in 
Antlioceros.  But  this  sporophyte  is,  of  course,  dependent  upon 
the  gametophyte  for  its  supply  of  water,  which  is  taken  up 
through  the  pointed  foot  of  the  stalk  that  is  deeply  sunken 
in  the  top  of  the  leafy  moss  plant  (Fig.  264,  B}.  The  spores 


sc 


B 


E 


FIG.  270.  The  sporophyte  of  a  common  moss  (Funaria) 

A,  young  sporophyte  s  attached  to  the  leafy  moss  plant  and  covered  by  the  calyptra 
ml.  />,  sporophyte  with  mature  spore  case  sc  and  calyptra  cat  at  the  tip.  C, 
spore  case  with  calyptra  removed :  o,  the  cover  (operculum).  D,  a  stoma  from 
the  surface  of  the  spore  case.  E,  section  of  young  spore  case,  showing  the 
cylindrical  central  region  of  spore-producing  tissue  sp.  f\  the  spore-producing 
tissue  in  detail.  —  Adapted  after  Campbell 

are  developed  in  groups  of  four  (tetrads)  within  spore  mother 
cells  (Fig.  270,  F,  sp),  which  form  a  barrel-shaped  tissue  (Fig. 
270,  E,  sp)  within  the  spore  case. 

In  spite  of  the  immense  numbers  of  species  in  the  Bryales, 
the  order  is  clearly  separated  from  other  groups  of  bryophytes 
as  a  side  line  of  plant  evolution,  and  its  families  and  genera  are 
distinguished  by  relatively  minor  differences. 


302  THE   BRYOPHYTES 

THE  ORIGIN  AND  EVOLUTION  OF  THE  BRYOPHYTES 

297.  The  origin  of  the  bryophytes.    The  origin  of  the  bryo- 
phytes  is  a  mystery.    They  have  of  course  arisen  from  the  algae, 
but  there  are  no  living  algae  that  resemble  the  bryophytes  at  all 
closely.    Coleochcete 1  and  the  stoneworts  (Char ales)  are  the  types 
most  frequently  considered  in  relation  to  the  mosses.    But  the 
sexual  organs  of  Coleochcete  are  one-celled,  and  the  female  organ 
of  Chara  bears  only  a  superficial  resemblance  to  an  archegonium, 
while  its  antheridium  is  totally  unlike  any  other  male  organ. 
There  must  have  been  formerly  some  group  of  the  algae,  prob- 
ably in  the  Chloropliycece,  distinguished  by  having  many-celled 
sexual  organs  from  which  the  antheridium  and  archegonium  of 
the  bryophytes  arose ;  for  these  complex  sexual  organs,  together 
with  the  characteristic  sporophyte  generation,  constitute  the  chief 
advance  of  the  bryophytes  over  the  algae. 

298.  The  evolution  of  the   bryophytes.    The   evolution  of 
the  bryophytes  is  clearly  related  to  the  change  from  the  aquatic 
habits  of  the  algae  to  the  land  habit.    Living  upon  the  land  exposes 
the  plant  to  the  drying  effects  of  the  air  and  demands  at  once  im- 
portant structural  adaptations,  that  is  to  say  the  plant  must  either 
develop  a  firm  cell  structure  so  that  drying  up  will  not  injure 
the  tissues  seriously,  or  else  it  must  maintain  a  constant  con- 
nection with  water  through  the  surfaces  of  filaments  (rhizoids) 
which   are   directly   in   contact   with   moisture.    Many   of  the 
mosses  and  leafy  liverworts  have  solved  the  problem  in  the  first 
way  and  may  become  quite  dry  without  serious  injury.    The  Ric- 
ciz  and  Marchantia  forms  and  Anthoceros,  on  the  other  hand, 
are  clearly  adapted  to  the  second  alternative  and  die  at  once 
if  removed  from  water  or  moist  earth.    The  creeping  habit  and 
thallus  structure  of  the  simpler  liverworts,  while  of  advantage 

1  The  fructification  of  Coleich&te  has  frequently  been  compared  to  a  sim- 
ple type  of  sporophyte,  somewhat  like  those  found  in  the  Riccia  group  of  the 
liverworts.  Recent  investigations,  however,  indicate  that  this  comparison 
is  not  justified,  and  that  the  fructification  is  not  sporophytic  at  all.  See 
Sec.  336  on  the  origin  of  the  sporophyte. 


SUMMARY  OF   THE  BRYOPHYTES  303 

in  some  situations,  d,o  not  constitute  so  effective  a  plant  body  as 
a  leafy  stem  with  an  erect  habit,  which  secures  a  much  greater 
exposure  to  air  and  light.  Accordingly  the  appearance  of  leafy 
stems  marked  a  great  advance  over  the  thallus  structure.  This 
new  form  of  bryophyte  plant  body  reached  its  highest  develop- 
ment when  the  stem  became  erect  with  the  leaves  arranged 
spirally,  as  in  the  mosses,  so  as  to  give  a  radial  symmetry. 

It  is  quite  safe  to  say  that  the  adoption  of  the  land  habit 
was  the  chief  cause  of  the  rapid  advance  of  the  bryophytes  over 
the  algse.  The  advance  in  vegetative  structure  is  generally  most 
marked  in  the  gametophyte  phase  of  the  life  history,  although 
the  sporophytes  of  such  types  as  Anthoceros  and  certain  mosses 
are  clearly  higher  than  the  gametophytes.  It  may  be  noted  in 
this  connection  that  the  next  great  forward  step  in  the  evolu- 
tion of  plants  came  in  the  fern  group,  or  pteridophytes,  when 
the  sporophyte  generation  adopted  the  land  habit  and  became 
independent  of  the  gametophyte.  However,  this  subject  will  be 
taken  up  in  the  next  chapter. 

SUMMARY  OF  THE  BRYOPHYTES  AND 
THALLOPHYTES 

299.  Bryophytes  and  thallophytes  compared.    It  is  possible 
at  this  point  to  make  clear  the  fundamental  reasons  for  the 
separation  of  the  spore  plants,  so  far  studied,  into  the  two  great 
divisions  of  the  plant  kingdom  called  the  Thallophyta  and  Bryo- 
phyta.    It  will  be  seen  that  the  bryophytes  have  a  set  of  very 
clearly  denned  characters,  while  the  thallophytes  are  distin- 
guished largely  by  the  absence  of  these. 

300.  Summary  of  the  bryophytes.    The  sexual  organs  are 
many-celled  structures  differentiated  into  f emaje  organs  (arche- 
gonia)  and  male  organs  (antheridia).    The  fertilized  egg  develops 
at  once  into  an  asexual  generation,  or  sporophyte,  which  pro- 
duces asexual  spores  in  groups  of  four,  or  tetrads,  within  cer- 
tain cells  called  spore  mother  cells.    The  sporophyte,  often  called 


304  THE  BRYOPHYTES 

the  fruit,  alternates  with  the  sexual  generation,  or  gametophyte, 
and  is  always  attached  to  it  and  dependent  upon  it  for  water 
and,  at  least  in  large  part,  for  certain  foods.  The  asexual  spores 
produced  by  the  sporophyte  are  of  a  new  type  not  found  in  the 
thallophytes. 

Class  I.  The  liverworts,  or  Hepaticce.  This  class  is  character- 
ized by  relatively  simple  sporophytes  (Antlioccros  excepted). 
The  gametophytes  are  thalloid  except  in  the  leafy  liverworts, 
and  have  distinct  upper  and  lower  surfaces  (dorsiventral 
symmetry  V 

Class  II.  The  mosses,  or  Musci.  These  have  relatively  com- 
plex sporophytes,  whose  spore  cases  open  by  covers,  and  the 
rim  of  the  spore  case  is  frequently  fringed  by  a  circle  of  teeth. 
The  gametophytes  have  erect  leafy  stems,  and  the  leaves  are 
generally  arranged  spirally  (radial  symmetry). 

301.  Summary  of  the  thallophytes.  The  sexual  organs  are 
almost  always  one-celled  structures.  The  chief  exceptions  are 
the  so-called  plurilocular  sporangia  of  the  brown  algse  (Sec.  235) 
and  the  peculiar  antheridium  of  the  stoneworts  (Sec.  230). 
There  is  no  organ  in  the  thallophytes  resembling  the  archegoriium 
in  structure  or  development.  There  is  no  alternation  of  sexual 
generations  with  asexual  in  most  of  the  thallophytes.  However, 
in  the  red  algse  (Rhodophycece)  and  the  sac  fungi  (Ascomycetes) 
the  fertilized  female  cell  produces  peculiar  fructifications  called 
cystocarps  and  ascocarps,  which  develop  asexual  spores  and 
constitute  phases  in  the  life  history,  alternating  with  the  sexual 
plants.  These  phases  are  sporophytes,  and  there  is  a  true  alter- 
nation of  generations  in  the  red  algse  and  sac  fungi,  but  these 
structures  are  peculiar  and  are  believed  to  be  independent  de- 
velopments in  these  two  remarkable  groups  and  not  related  to 
the  sporophytes  of  the  bryophytes.  None  of  the  thallophytes 
have  sexual  plants  resembling  in  detail  those  of  the  liverworts  or 
mosses.  The  plant  body  is  generally  a  thallus,  though  the  variety 
of  form  is  very  great,  but  the  highest  types  in  the  brown  and 
red  algse  are  differentiated  into  stems,  leaf-like  structures,  and 


SUMMARY  OF  THE  THALLOPHYTES  305 

holdfasts.  The  cell  structure  of  the  thallophytes  is  generally 
much  simpler  than  that  in  the  plant  bodies  of  the  bryophytes, 
which  owe  their  complexities  of  cell  structure  chiefly  to  the 
varied  conditions  introduced  by  the  land  habit;  for  the  land 
habit  requires  the  plant  to  protect  itself  from  drying  up,  in 
the  air.  This,  in  general,  means  that  a  land  plant  must  obtain 
water  from  the  soil  through  some  kind  of  organs  adapted  for  that 
purpose  (rhizoids  or  roots).  And,  as  a  rule,  a  land  plant  soon 
differentiates  a  protective  layer  of  cells  (epidermis),  which  helps 
to  hold  the  water  within  its  tissues.  These  structures  are  either 
entirely  absent  or  present  in  greatly  reduced  form  in  aquatic 
plants,  and  for  these  reasons  the  cell  structure  of  the  aquatic 
thallophytes  is  generally  very  much  simpler  than  that  of  the 
bryophytes. 

Nevertheless,  the  thallophytes  have  developed  some  compli- 
cated organs  with  highly  differentiated  tissues,  as  in  the  kelps, 
rockweeds,  red  algae,  sac  fungi,  and  the  higher  basidia  fungi, 
such  as  the  toadstools  and  mushrooms,  puffballs,  nest  fungi, 
and  carrion  fungi.  These  complexities  are,  however,  very  spe- 
cial in  character  and  not  related  to  the  structure  of  higher 
groups  of  plants. 


CHAPTEK  XXV 

THE    PTERIDOPHYTES    AND    THE    APPEARANCE    OF 
HETEROSPORY 

302.  The  pteridophytes.*    The  division  Pteridopliyta  (mean- 
ing fern  plants)  comprises  three  classes :  (1)  the  ferns,  or  Fili- 
cinece,  (2)  the  horsetails,  or  Equisetinece,  and  (3)  the  club  mosses, 
or  Lycopodinece.    Representatives  of  these  groups  are  generally 
somewhat  familiar  to  all,  and  no  one  would  think  of  grouping 
them  with  the  liverworts  and  mosses.    The  differences  become 
more  conspicuous  after  a  study  of  the  life  histories  of  pteri- 
dophytes, which  shows  that  the  large  fern  plant  with  its  roots, 
stem,  and  leaves  is  really  an  asexual  generation,  or  sporophyte, 
and  that  the  gametophyte  is  represented  by  a  small,  compara- 
tively insignificant  sexual  generation.    This  condition,  so  dif- 
ferent   from    anything   in   the    bryophytes    and   thallophytes, 
marks  one  of  the  great  forward  steps  in  the  progress  of  plant 
evolution.    It  leads  towards  the  seed  plants,  for  these  highest 
forms  with  their  varied  and  complex  structures  are  sporophytes, 
whose  gametophyte  generations  are  so  much  reduced  that  they 
can  only  be  recognized  by  careful  study  of  the  processes  of  seed 
formation. 

303.  The   advances   in  plant   evolution   up   to   the   pteri- 
dophytes.   It  is  well  to  summarize  at  this  point  the  contribu- 
tions of  the  thallophytes  and  bryophytes   to   the  progress  of 
plant  evolution. 

1.  The  algce.  The  chief  contributions  of  the  algre  to  plant 
evolution  were  four  in  number:  (1)  the  attached  many-celled 
plant  body  arose  from  the  single-celled  condition  of  the  lowest 

*  To  THE  INSTRUCTOR  :  The  introduction  to  this  chapter  assumes  that  the 
life  history  of  some  fern  has  been  studied  in  the  laboratory. 

306 


THE  ADVANCE   IN   PLANT  EVOLUTION  307 

alg?e  and  soon  became  established  as  the  vegetative  period  in 
the  plant's  life  history ;  (2)  as  a  result  of  this,  the  motile 
stages  (zobspores)  became  set  apart  as  reproductive  phases  in 
the  life  histories,  such  reproductive  motile  stages,  with  other 
reproductive  cells,  being  called  spores ;  (3)  certain  of  the  repro- 
ductive cells  became  sexual  in  character,  and  these  gametes  at 
first  similar  (isogamy)  were  later  differentiated  into  eggs  and 
sperms  (heterogamy) ;  (4)  alternation  of  generations  developed 
'in  the  red  algae  and  sac  fungi,  but  probably  independently  of 
the  same  phenomenon  in  the  bryophytes. 

2.  The  fungi.    The  fungi  as  special  and  peculiar  offshoots 
from  the  algae  have  of  course  contributed  nothing  to  the  main 
evolutionary  line  running  up  to  the  higher  plants. 

3.  The  liverworts  and  mosses.    The  chief  advances  of  the 
liverworts  and  mosses  over  the  algae  were  three  in  number: 

(1)  many-celled  sexual  organs  (antheridia  and  archegonia)  took 
the  place  of  the  one-celled  reproductive  organs   of  the  algse; 

(2)  an    alternation    of    generations    (gametophyte    with    sporo- 
phyte)  became  well  established,  together  with  the  origin  of  a 
new  type  of  asexual  spore  developed  in  groups  of  four  (tetrads) 
by  the  sporophyte ;  (3)  there  was  a  general  advance  in  the  cell 
structure  of  the  plant  bodies  because  of  adaptations  to  the  more 
complex  conditions  of  the  land  habit. 

The  sporophyte  of  the  bryophytes  is  always  attached  to  the 
gametophyte,  and  except  in  Anthoceros  and  some  mosses  it  is 
not  as  complex  as  the  gametophyte.  In  the  pteridophytes,  how- 
ever, the  conditions  are  reversed.  The  sporophyte  is  the  large, 
conspicuous  phase  in  the  life  history,  and  as  it  develops  it  becomes 
entirely  independent  of  the  gametophyte,  while  the  latter  appears 
relatively  insignificant,  although  it  holds,  of  course,  a  necessary 
place  in  the  life  history.  The  appearance  in  the  pteridophytes 
of  the  sporophyte  as  an  independent  plant  was  the  most  impor- 
tant advance  in  plant  evolution  at  this  time,  for  the  vegetative 
activities  gradually  became  shifted,  at  first  chiefly  and  finally 
wholly,  from  the  sexual  to  the  asexual  generation. 


FIG.  271.   A  fern  (Aspidium  Filix-mas) 

A,  part  of  the  creeping  stem,  or  rootstock,  and  fronds:  fr,  young  fronds  unrolling. 
B,  under  side  of  a  frond,  showing  sori  s.  C,  section  through  a  sorus  at  right 
angles  to  surface  of  the  leaf,  showing  indusiura  i  and  sporangia  s.  D,  a  spo- 
rangium discharging  spores.  —  After  Wossidlo 


THE  FERNS 


309 


The    material  of   this    chapter   will   be    treated    under   the 
following  headings: 

Class  I.       The  ferns,  or  Filicinece. 

Class  II.     The  horsetails,  or  Equisetinece. 

Class  III.  The  club  mosses,  or  Lycopodineat. 

Fossil  plants  and  coal. 

The  origin  and  evolution  of  the  pteridophytes. 

Summary  of  the  pteridophytes  and  their  advances  over  the  bryophytes. 


CLASS  I.   THE  FEKNS,  OR  FILICINEM 

304.  The  ferns.  The  ferns  are  a  very  large  assemblage  of 
more  than  four  thousand  species,  and  most  of  them  can  be 
recognized  at  a  glance  by 
the  characteristic  forms  of 
their  leaves,  called  fronds, 
and  by  their  habits  of 
growth.  They  are  widely 
distributed,  but  reach  their 
greatest  luxuriance  in  the 
tropics,  where  they  present 
some  very  striking  dis- 
plays. Thus  the  tree  ferns 
have  stems  thirty  or  forty 
feet  high,  with  a  crown  of 
fronds  often  fifteen  or  more 
feet  in  length.  The  stems 
of  some  of  the  tree  ferns 
are  covered  by  a  sheath  of 
fibrous  roots  (as  in  Dick- 
sonia,  Plate  VII),  and  in 
other  types  the  bases  of  FlG<  272<  The  stag.horn  fern  (piatyCcrium 
the  old  and  withered  fronds  Willinki) 

form  a  similar  investment.    A  tropical  epiphytic  fern  with  two  forms  of 

There  are  also  in  the  tropics      lea ves'  °ne  ?f  which  grows  close]5r  a§ainst 

the  bark  of  trees  and  gathers  and   holds 
Certain  Small  delicate  ferns        moisture  and  humus.  —  After  Goebel 


310 


THE  PTERIDOPHYTES 


called  filmy  ferns,  whose  stems  and  fronds  are  as  delicate  as 
mosses.  Some  peculiar  types,  as  the  stag-horn  fern  (Platy cerium, 
Figs.  272,  364),  grow  over  the  surface  of  tropical  trees  and  are 
consequently  called  epiphytes  (meaning  upon  a  plant).  These  have 
certain  flattened  leaves  (Fig.  2 72)  closely  pressed  against  the  sur- 
face to  which  the  plants  are  attached,  where  they  gather  and  hold 
moisture  and  humus.  The  roots  of  the  filmy 
ferns  and  the  epiphytes  are  very  poorly  devel- 
oped, or  entirely  wanting,  but  the  air  of  a  tropi- 
cal forest  is  so  saturated  with  water  that  they 
obtain  all  that  they  need  from  the  dripping  mois- 
ture and  wet  surfaces  upon  which  they  grow. 

All  the  plants  which  one  would  at  a  glance 
call  ferns  are  sporophytes,  and  they  consequently 
produce  asexual  spores,  which  are  borne  upon 
their  fronds.  These  spores  correspond  exactly 
to  those  developed  by  the  sporophytes  of  the 


FIG.  273.   The  walking  fern 
(Camplostrus  rhizophyllus) 


Showing  the  manner  in  which 
fronds  hearing  reproductive 
buds  at  their  tips  bend  over 
and  establish  new  plants 

liverworts  and  mosses,  and  they  give  rise  to  a  small  sexual 
generation,  the  gametophyte.  The  life  history  of  a  pteridophyte 
can  be  most  easily  studied  from  one  of  the  common  ferns  famil- 
iar to  us  in  the  woods,  greenhouses,  and  gardens.1 

i  The  moonwort  and  adderVtongue  (Sec.  315)  illustrate  more  primitive 
conditions  in  the  pteridophy tes  than  the  common  ferns,  but  are  not  generally 
available  for  type  studies. 


PLATE  VII.    Tree  ferns  (Dicksonia  antarctica)  from  Tasmania 

These  tree  ferns  grow  to  be  30  to  40  feet  high,  with  fronds  8  to  12  feet  long,  and  the 
trunks,  densely  covered  with  small  roots,  may  become  3  feet  thick.  —  After  a 
photograph  in  the  Harvard  Museum 


THE  COMMON   FERNS  311 

305.  The  common  ferns.  The  common  ferns  (order  Filicales) 
completely  outclass  all  other  orders  of  pteridophytes  in  num- 
ber of  species  and  mass  of  vegetation.  The  forms  are  exceed- 
ingly various.  The  stems  may  be  short  and  close  to  the  ground, 
or  upright  trunks,  as  in  the  tree  ferns.  But  many  types  have 
creeping  stems,  frequently  wholly  buried  in  the  earth  as  under- 
ground stems,  or  rootstocks,  well  illustrated  by  the  common  brake, 
or  bracken  fern  (Pteris  aquilina).  Some  ferns  have  peculiar 
methods  of  reproduction  by  buds  that  are  formed  on  the  leaves, 
as  in  the  bladder  fern  (Cystoptcris  bidbifera),  or  the  walking 
fern  (Camptosorus)  shown  in  Fig.  273. 

The  fronds  or  leaves  arise  from  the  tip  of  the  stem  and  form 
clusters  or  crowns  around  the  top  of  upright  structures,  but  are 
generally  somewhat  scattered  along  the  creeping  sterns.  Most 
fronds  are  much  cut  or  divided  (compound)  after  regular  and 
various  patterns  (Fig.  271).  They  are  developed  very  slowly 
in  some  genera,  remaining  rolled  up  in  the  bud  for  several 
months.  However,  when  fully  formed  and  in  the  proper  season 
they  unroll  comparatively  quickly  from  the  base  in  a  very 
characteristic  manner  until  the  apex  finally  appears  above. 

The  cell  structure  of  the  leaves,  steins,  and  roots  is  very 
much  more  complex  than  the  cell  structure  in  the  bryophy tes 
and  recalls  at  once  the  tissues  of  the  seed  plants  (see  Part  I, 
Chapters  vii  and  xn).  The  plant  body  has  a  system  of  tissues, 
called  fibro-vascular  bundles  (Fig.  274),  whose  parts  are  much 
modified  for  two  important  functions.  One  tissue  is  composed 
of  large  cells  (Fig.  274,  t)  empty  of  protoplasm  and  with  heavy 
thickened  walls  marked  with  curious  pits.  These  elements, 
called  traclieids,  compose  the  woody  part  of  the  fibro-vascular 
bundle  termed  the  xylem,  and  their  purpose  is  to  conduct  water 
from  the  roots  to  parts  of  the  plant  above  ground.  But  they 
are  also  very  important  for  the  strength  that  they  give  to  stems 
and  leaves.  Another  tissue  is  composed  chiefly  of  cells,  called 
sieve  tubes  (Fig.  274,  st),  which  contain  much  protoplasm  and 
food  material  and  make  up  a  softer  region  of  the  bundle  termed 


312  THE  PTERIDOPHYTES 

bast  or  phloem.  The  bast  regions  are  known  to  be  paths  for  the 
distribution  of  food  material  in  the  plant.  The  structure  of  the 
fern  frond  is  essentially  similar  to  that  of  the  leaf  of  a  seed 
plant.  There  are  stomata  on  the  lower  surface  and  chlorophyll- 
bearing  tissues  underneath  the  outer  cell  membrane,  or  epidermis. 
The  nbro-vascular  bundles  run  out  into  the  green  expanded  por- 
tions of  the  leaves  as  forking  veins,  which  do  not,  however, 


FIG.  274.  Fibro-vascular  bundle  from  the  underground  stem,  or  rootstock,  of 
the  common  brake  (Pteris  aquilina) 

g,  ground  tissue,  or  parenchyma;  6s,  bundle  sheath;  ps,  bast,  or  phloem,  sheath 
surrounding  the  sieve  tubes  (st)  and  bast  fibers  which  compose  the  bast,  or 
phloem ;  t,  large,  thick-walled  cells  called  tracheids,  which  with  smaller  cells 
in  the  center  make  up  the  wood,  or  xylem 

generally  unite  to  form  the  close  network  so  characteristic  of 
dicotyledonous  seed  plants. 

As  stated  above,  the  fibro- vascular  bundles  greatly  strengthen 
the  tissues  of  the  leaves  and  stems,  for  they  form  a  sort  of 
skeleton  in  the  plant.  They  are  frequently  assisted  in  their 
strengthening  functions,  especially  in  the  stem,  by  regions  of 
rigid  tissue,  which  may  be  variously  situated,  sometimes  under 


SPORE  FORMATION  313 

the  epidermis  and  sometimes  forming  broad  strands  in  the 
interior.  This  rigid  tissue  (selerenchyma)  is  composed  of  elon- 
gated cells  with  very  heavy,  much-thickened  walls,  which  are 
often  yellowish  in  color.  This  tissue  is  developed  from  the 
thin-walled  cells  (parenchyma),  called  the  ground  tissue,  that 
compose  the  greater  part  of  the  interior  of  the  stems. 

306.  Spore  formation.  The  sporophyte  nature  of  the  fern 
plant  Becomes  clear  at  the  time  of  fructification.  Certain  ones 
or  sometimes  all  of  the  fern  fronds  as  they  grow  older  develop 
spore  cases,  or  sporangia.  These  are  variously  situated  on  the 
fronds,  sometimes  appearing  as  clusters  or  spots,  called  sori 
(singular,  sorus,  meaning  a  heap),  on  the  under  surface  and  some- 
times in  lines  along  the  under  edge.  A  sorus  may  be  naked, 
but  it  is  frequently  protected  by  a  membranous  outgrowth,  or 
indusium,  from  the  surface  of  the  frond  (Fig.  271,  C,  i). 

The  sporangia  are  stalked  and  somewhat  flattened  many- 
celled  cases,  each  of  which  develops  from  a  single  surface  cell 
of  the  frond.  There  are  sixteen  spore  mother  cells  in  the  inte- 
rior of  the  spore  case,  each  of  which  gives  rise  to  a  group  of 
four  spores  (tetrad).  The  method  of  spore  formation,  four  spores 
in  each  mother  cell,  is  thus  identical  with  that  of  the  bryophytes. 

The  sporangium  of  many  common  ferns  is  composed  of  thin- 
walled  cells  except  along  the  edge,  where  there  is  a  line  with 
much-thickened  wralls,  which  extends  from  the  stalk  about  two 
thirds  around  on  the  outside  (Fig.  275,  A).  This  line  of  cells  is 
called  the  ring,  and  as  the  sporangium  ripens  and  becomes  dry, 
the  ring  is  forcibly  held  like  a  bent  spring.  Finally,  the  deli- 
cate cells  at  the  side  of  the  spore  case  opposite  the  ring  are 
unable  to  stand  the  strain  and  are  torn  apart  so  that  the  ring 
straightens  somewhat  and  a  wide  rent  is  made  in  the  side  of 
the  sporangium  (Fig.  275,  B,  C}.  The  spores  are  thrown  out  vio- 
lently through  the  rent  for  a  considerable  distance.  This  is  the 
structure  of  the  spore  case  in  the  family  Polypodiacem,  for  the 
several  families  of  the  Filicales  have  sporangia  which  differ  from 
one  another  in  form  and  in  the  structure  and  position  of  the  ring. 


314 


THE   PTERIDOPHYTES 


307.  Fronds,  vegetative  leaves,  and  spore  leaves  (sporo- 
phylls).  Most  fronds  are  vegetative,  that  is,  perform  chloro- 
phyll work  (photosynthesis)  during  the  early  part  of  the  season 
and  develop  sporangia  later.  However,  some  types,  as  the  royal 
and  cinnamon  ferns  (Osmunda)  and  the  sensitive  and  ostrich 
ferns  (Onoclea),  devote  the  whole  of  certain  leaves  or  portions  of 
them  entirely  to  the  work  of  spore  production.  The  blades  of 
these  fronds  or  portions  of  fronds  never  become  expanded,  but 
remain  somewhat  rolled  up,  forming  pod-like  structures  in  which 
the  sporangia  are  developed  (Fig.  276).  Other  fronds  on  these 


FIG.  275.   The  sporangium  of  a  common  fern  (Aspidium  Filix-mas) 

A,  closed  sporangium;  B,  sporangium  opening;  C,  fully  opened  and  discharging 
the  spores.  —  After  Kerner 

ferns  are  devoted  entirely  to  vegetative  activities  and  never  de- 
velop sporangia.  There  is  thus  in  some  ferns  a  division  of  labor 
among  the  fronds,  certain  of  them  becoming  strictly  vegetative 
leaves,  while  others  are  spore  leaves,  called  sporophylls. 

There  is  a  constant  tendency  in  the  pteridqphytes  to  give  all 
the  work  of  spore  production  to  the  specialized  spore  leaves 
(sporophylls),  which  means  that  all  the  other  fronds  on  the 
plant  become  entirely  devoted  to  vegetative  activities  and  may 
then  be  called  vegetative  leaves  or  simply  leaves  in  the  sense  in 
which  this  term  is  generally  used  in  the  seed  plants.  This 
differentiation  of  the  frond  into  leaves  arid  sporophylls  reaches 


THE   GAMETOPHYTE   OF   THE   FERN 


315 


a  high  point  of  development  in 
the  horsetails  and  club  mosses, 
and  becomes  even  more  con- 
spicuous in  the  seed  plants,  as 
will  appear  later. 

308.  The  gametophyte  of  the 
fern.  The  fern  spore  germinates 
readily  on  moist  surfaces  and 
puts  forth  a  delicate  filament, 
consisting  of  a  row  of  cells  (Fig. 
277,  A).  Several  oblique  cell 
walls  at  the  end  of  this  filament 
cut  out  a  triangular  apical  cell 
(Fig.  277,  B,  x),  which  becomes 
the  growing  point.  The  final  de- 
velopment is  usually  a  small, 
delicate,  heart-shaped,  thallus- 
like  body  resembling  a  small 
liverwort,  but  only  one  cell  in 
thickness,  except  in  the  middle 
region.  The  apical  cell  (Fig.  277, 
C,  x)  generally  becomes  situated 
in  a  deep  notch  at  the  forward 
end  (Fig.  277,D)  because  of  the 
greater  cell  growth  on  either  side. 
The  back  part  of  the  structure 
becomes  fastened  to  the  earth  by 
numerous  delicate  filaments  or 
rhizoids  which  act  like  root  hairs. 
This  structure  develops  the  sex- 
ual organs  of  the  fern  and  is  con- 
sequently the  gametophyte  gener- 
ation in  the  life  history.  It  is  called 


FIG.  276.  The  sensitive  fern  (Onoclea 
sensibilis) 


the  prothallium  because  it   pre-    Showing  vegetative  leaf  and  spore  leaf 

(sporophyll)  rising  from  the  creeping 

cedes  the  fern  plant  (sporophyte).      rootstock 


316 


THE  PTERIDOPHYTES 


Both  sexual  organs  (antheridia  and  archegonia)  are  found  on 
the  same  prothallium  if  it  is  well  developed.  But  when  pro- 
thallia  are  crowded  or  grown  under  other  unfavorable  conditions 
they  remain  small  and  stunted  and  become  irregular  in  form 
(Fig.  278,  A).  Such  dwarf  gametophytes  only  develop  antheridia. 


FIG.  277.   The  fern  prothallium  and  archegonium 

A,  stages  in  the  germination  of  the  spore.  B,  young  prothallium,  showing  first 
/  appearance  of  wedged-shaped,  apical  cell  x,  C,  tip  of  prothallium  beginning 
to  take  on  the  heart-shaped  form:  x,  apical  cell.  Z),  mature  prothallium, 
showing  group  of  archegonia  on  the  cushion  just  hack  of  the  notch,  and  anther- 
idia further  back :  rh,  rhizoids.  E,  an  open  archegonium  with  egg  ready  for 
fertilization,  and  two  sperms  near  the  entrance  of  the  neck.  —  A,  B,  C,  E,  after 
Campbell;  D,  after  Schenck 

On  well-nourished  prothallia  the  antheridia  are  formed  first  on 
the  edge  and  lower  surface  of  the  back  portions.  The  arche- 
gonia are  developed  last  when  the  prothallium  is  quite  large,  and 
are  only  found  on  the  thickened  region,  called  the  cushion, 
directly  back  of  the  notch,  or  growing  point. 

The  antheridia.   The  antheridia  (Fig.  278,  B,  C}  are  very  much 
smaller  than  those  of  the  bryophytes.    They  develop  from  a 


THE   GAMETOPHYTE   OF  THE  FERN 


317 


single  cell  which  projects  above  the  surface  of  the  prothallus. 
There  are  only  three  cells  forming  the  capsule  of  this  structure, 
—  a  cover  cell  above,  a  ring-shaped  cell  in  the  middle,  and  a 
funnel-shaped,  basal  cell.  These  three  cells  inclose  at  first  a  large 
central  protoplast,  from  which  is  developed  a  group  of  one  hun- 
dred or  more  small  cubical  cells 
that  produce  the  sperms,  as  in  the 
bryophytes.  These  sperms  are,  how- 
ever, very  different  in  form  from 
the  two-ciliate  sperms  of  the  liver- 
worts and  mosses  and  many  algse. 
Each  consists  of  a  spirally  coiled 
band  (Fig.  278,  Z>),  whose  narrower- 
pointed  end  is  covered  with  numer- 
ous cilia,  making  it  a  many-ciliate 
sperm. 

The  archegonia.  The  archegonia 
of  the  ferns  are  also  much  smaller 
than  those  of  the  bryophytes  and 
simpler  in  structure.  The  short 
neck  alone  projects  above  the  sur- 
face of  the  prothallus  (Fig.  277,  E) 
and  generally  bends  backward, 
probably  because  the  forward  part 
of  the  prothallium  is  not  directly 
on  the  earth,  but  rises  at  an  angle. 
The  egg  lies  beneath  the  surface  of 
the  prothallium,  so  that  the  base 
of  the  archegonium  may  be  described  as  sunken.  There  are 
only  two  or  three  canal  cells  (Sec.  283)  in  this  archegonium. 

The  eggs  are  fertilized  under  exactly  the  same  conditions  as 
in  the  bryophytes  (Sec.  283).  When  the  prothallia  are  wet  the 
sexual  organs  open,  and  the  sperms  swim  over  the  moist  surfaces 
and  are  attracted  to  the  necks  of  the  archegonia  by  substances 
secreted  within,  one  of  which  at  least  is  malic  acid.  The 


FIG.  278.  The  antheridium  and 
sperms  of  a  fern  (Onoclea) 

A,  small  prothallium  with  many 
antheridia  an :  s,  old  spore  wall. 
B,  antheridium,  showing  cover 
cell  c,  ring  cell  r,  and  basal  cell 
b,  inclosing  the  sperm  mother 
cells.  C,  antheridium  opening. 
D,  sperms.  —  After  Campbell 


318 


THE  PTERIDOPHYTES 


sperms  swim  down  the  neck  to  the  egg,  and  one  of  them  fer- 
tilizes it.  The  fern  plant  then,  like  the  liverwort  and  moss, 
practically  returns  to  the  aquatic  life  of  the  algae  at  the  time 
when  the  sexual  cells  are  functional. 

309.  The  development  of  the  sporophyte.    The  early  stages  in 
the  development  of  the  fern  sporophyte  as  in  the  bryophytes 


rh 


FIG.  279.    Development  of  the  sporo- 
phyte of  a  fern 

A,  section  of  prothallium  with  a  young  sporo- 
phyte :  c,  thickened  region,  or  cushion,  in  which 
is  imbedded  the  foot;  /,  first  leaf;  r,  root;  ar, 
unfertilized  archegonia;  an,  old  antheridia; 
rh,  rhizoids.  B,  an  old  prothallium  with  young 
fern  sporophyte  attached,  whose  first  leaf  I  has 
grown  up  through  the  notch  at  the  forward  end 
of  the  prothallium,  while  the  root  r  has  entered 
the  earth :  rh,  rhizoids.  —  After  Sachs 

are  passed  entirely  within  the  tissue 
of  the  prothallium,  surrounded  by  the 
remains  of  the  archegonium.  The  ferti- 
lized egg  cell  divides,  and  there  are 
formed  four  regions  in  the  embryo 
fern  :  (1)  a  stem  region,  (2)  the  first  leaf,  (3)  the  first  root,  and 
(4)  an  organ  of  attachment  to  the  gametophyte  called  the  foot. 
The  leaf  and  root  soon  break  out  of  the  archegonium,  the  first 
growing  upward  and  the  second  into  the  earth  (Fig.  279,  A,  B). 
The  stem  grows  more  slowly.  The  young  fern  all  this  time 
obtains  nourishment  from  the  prothallium  through  the  foot  after 
exactly  the  same  method  as  in  the  bryophytes.  However,  when 


rh 


SUMMARY  OF  THE  LIFE  HISTORY  OF  A  FERN      319 

the  root  and  leaf  are  well  established  the  sporophyte  becomes 
independent  of  the  gametophyte,  which  gradually  dies  within 
a  few  weeks  or  months.  It  is  the  development  of  root,  stem, 
and  leaf  on  the  part  of  the  sporophyte,  giving  it  complete  inde- 
pendence, which  marks  the  greatest  advance  of  the  pteridophytes 
over  the  bryophytes. 

310.  Summary  of  the  life  history  of  a  fern.  The  alterna- 
tion of  generations  in  the  fern  is  much  more  apparent  than  in 
the  liverworts  and  mosses  because  both  gametophyte  and  sporo- 
phyte are  independent  plants.  The  two  groups  (bryophytes  and 
pteridophytes)  are  in  striking  contrast  in  the  relative  importance 
of  the  two  generations.  The  gametophytes  of  the  bryophytes 
are  relatively  large,  long-lived,  and  complex  organisms  (with 
stems  and  leaves  in  the  mosses  and  leafy  liverworts),  while  the 
sporophytes  are  simple  and  so  dependent  upon  the  gametophyte 
that  they  were  for  many  years  called  its  fruit.  The  gameto- 
phytes of  the  pteridophytes,  on  the  contrary,  are  small,  short- 
lived, and  simple,  while  the  sporophytes  are  very  large  and 
complex  (possessing  stem,  roots,  .leaves,  and  a  vascular  system) 
and,  except  in  their  earliest  stages  of  development,  completely 
independent  of  the  gametophytes. 

The  life  history  of  a  fern  may  be  formulated  as  follows  : 

Gametophyte  (prothallium)  <^  ^        >  —  Sporophyte  (fern  plant) 

*J  u 

—  asexual  spores  —  Gametophyte,  etc. 
This  in  abbreviated  form  becomes 

-  S  —  sp  —  G,  etc., 


and  is  the  same  life-history  formula  as  that  of  the  bryophytes 
(Sec.  285). 

311.  Apogamy  and  apospory  in  the  ferns.  There  are  some 
irregularities  in  the  life  histories  of  certain  ferns  which  are 
not  uncommon  in  greenhouses  and  under  cultivation  (species 
of  Pteris,  Aspidium,  Athyrium,  Nephrodium,  etc.).  Prothallia 


320 


THE  PTERIDOPHYTES 


sometime  fail  to  develop  archegonia  or  the  archegonia  do 
not  function,  but  the  sporophyte  generation  arises  as  a  bud-like 
outgrowth  from  the  prothallium.  In  other  cases  the  egg  may 
develop  without  fertilization  (parthenogenesis).  Such  suppres- 
sions of  sexuality  with  the  development  of  a  succeeding  genera- 
tion asexually  are  called  apogamy.  The  phenomenon  has  been 
noted  before  in  the  water  molds  (Sec.  262)  and  other  fungi,  and 
it  is  found  in  various  groups  throughout  the  plant  kingdom. 

Apospory  is  the  suppression  of  the  process  of  spore  formation 
and  the  development  of  a  gametophyte  generation  directly  from 
the  sporophyte.  It  is  found  in  many  of  the  ferns,  which  are  also 
apogamous,  and  is  shown  by  the  presence  of  prothallia,  which 
are  direct  outgrowths  from  the  fern  frond 
in  the  place  of  the  sporangia,  or  sometimes 
at  the  tips.  Apospory  is  also  found  in 
certain  mosses  where  protonema  may 
develop  directly  from  portions  of  the  stalk 
and  spore  cases  of  the  sporophytes. 

Apogamy  and  apospory  are  both  short 
cuts  in  the  life  histories,  which  are  believed 
to  be  due  to  some  unusual  life  conditions 
that  interfere  with  the  regular  develop- 
ment of  gametes  and  spores  in  the  normal 
life  histories,  established  during  the  evolu- 
tion of  plant  groups. 

312.  The  water  ferns.  The  water  ferns 
(order  Hydropterales]  include  four  inter- 
esting genera  (Marsilia,  Pilularia,  Sal- 
and  Azolla},  each  of  which  is 
for  some  peculiarity  of  struc- 
ture. Salvinia  (Fig.  280)  and  Azolla  are 
floating  aquatics,  and  Marsilia  and 
Pilularia  are  either  aquatic  or  grow  in  very  wet  places.  These 
habits  give  the  common  name  of  water  ferns  to  the  group.  They 
are  important  illustrations  of  the  condition  called  lieterospory, 


FIG.  280.   A  water  fern 
(Salvinia) 

/,  floating  leaves;  r,  highly 
modified  leaf  acting  as  a 
root;  s,  spore  fruits. — 
After  Pringsheim 


THE  WATER  FERNS 


321 


which  is  briefly  described  in 
Sec.  214  and  discussed  in 
some  detail  in  Chapter  XXVIL 
The  Hydropterales  are  be- 
lieved to  have  been  derived 
from  the  Filicales,  and  the 
development  of  heterospory  is  so 
the  most  important  advance 
over  that  group.  We  can  only 
consider  the  rather  widely 
distributed  type  Marsilia. 

313.  Marsilia.*  Marsilia, 
the  clover  leaf  fern,  or  pepper- 
wort,  is  easily  recognized  from 
the  form  of  the  leaf  (Fig. 
281,  A).  The  leaves  arise 
from  a  creeping  stem  which 
in  certain  species,  as  M. 
quadrifolia,  grow  over  the 
mud  in  shallow  water  along 
the  margins  of  ponds  and 
streams,  but  often  come  out 
of  the  water  upon  muddy 
banks.  Other  species,  as  M. 
vestita,  grow  almost  entirely 
on  muddy  banks  or  in  wet 
meadows. 

The  spores  of  Marsilia  are 
developed  in  bean-shaped 


FIG.  281.   Marsilia 


A,  creeping  stem  of  Marsilia  quadrifolia, 
showing  a  series  of  leaves  in  various 
stages  of  development:  s,  spore  fruits 
(sporocarps).  B,  a  spore  fruit  of 
M.  vestita,  which  has  opened  in  water 
and  extruded  a  gelatinous,  worm-like 
structure  bearing  sori  so 


*  To  THE  INSTRUCTOR  :  If  only 
one  heterosporous  pteridophyte  can 
be  studied  in  the  laboratory,  it  is 
much  better  that  the  type  be  Sela- 
ginella.  For  this  reason  the  ac- 
count of  Marsilia  has  been  made  short.  The  life-history  formula  is,  of 
course,  the  same  as  that  of  Selaginella,  which  is  fully  treated  in  Sec.  326. 


322 


THE    PTERIDOPHYTES 


spore  fruits,  or  sporocarps,  borne  in  groups   on   short   stalks. 

These  spore  fruits  (Fig.  281,  A,  s)  are  really  modified  portions  of 

leaves,  which  are  excellent  illus- 
trations of  very  highly  developed 
sporophylls,  much  more  special- 
ized than  the  spore  leaves  of  such 
ferns  as  Onoclea  and  Osmunda. 

The  spore  fruits  burst  open 
when  soaked  in  water  through 
the  swelling  of  mucilage  within, 
and  the  contents  come  out  as  a 
gelatinous,  worm-like  structure 
bearing  large  groups  (sori)  of  spores 
along  the  sides  (Fig.  281,  B,  so). 
The  spores  are  developed  in  es- 
sentially the  same  manner  as  in 
the  common  ferns  (Filicales),  but 
the  tissues  of  the  sporangia  are 
so  much  modified  that  the  re- 
semblances can  only  be  followed 
through  a  detailed  developmental 
study.  The  spores  are  set  free 
by  the  softening  of  the  gelatinous 
material/and  they  begin  to  germi- 

A,  male  gametophyte  within  micro-  •       i  ? 

spore:  p,  prothaiiial  cells;   two   nate  at  once  in  the  water.    -They 

groups  of  sperm  mother  cells  shown  f  fc         &[          k          &nd  gmall 

within.    B,  sperms.    C,  female 

gametophyte  consisting  of  a  single   and  are  consequently  called  mega- 

archegonium    at  one   end  of    the    8poreS  Bud  microtpores.     This  con- 
megaspore,   which    is    filled   with       " 

starch  grains.  D,  a  week-old  em-   dition  is  termed  lieterospory. 

bryo  (slightly  magnified)  still  at-          ^.         mpo-oonnrpq     are     full     of 
tached  to  the  megaspore:    I,  first 

leaf;  r,  root.  —A,  B,  after  Camp-   starch  grains,  which  furnish  the 

food   for  the  development    in    a 

few  hours  of  a  small  female  gametophyte.  This  gametophyte 
(Fig.  282,  C)  consists  of  a  single  archegonium  at  one  end  of  the 
spore.  Although  the  cells  are  somewhat  greenish,  it  is  perfectly 


FIG.  282.   Gametophytes  of 
Marsilia  vestita 


MARSILIA  323 

clear  that  the  food  of  this  much-reduced  prothallium  is  fur- 
nished chiefly  by  the  sporophyte  by  means  of  the  megaspore. 
The  gametophyte  of  Marsilia  has  therefore  degenerated  from 
the  independent  condition  in  the  common  ferns  and  is  now 
no  longer  self-supporting,  but  is  dependent  upon  food  stored  by 
the  sporophyte,  a  relation  which  is  exactly  the  reverse  of  that 
in  the  bryophytes. 

The  microspore  develops  a  very  small  male  gametophyte  even 
more  quickly  than  the  megaspore  develops  the  female  one.  This 
structure  consists  of  a  lens-shaped  sterile  cell  called  the  prothal- 
lial  cell  (Fig.  282,  A,  p),  together  with  a  group  of  cells  which 
probably  represent  a  single  much-reduced  antheridium.  The 
sperms  are  formed  within  this  group.  They  are  remarkably  long, 
coiled  bands  covered  with  cilia  (Fig.  282,  B),  and  are  among  the 
largest  sperms  in  the  pteridophytes. 

The  young  sporophyte  develops  within  the  archegonium,  fol- 
lowing essentially  the  same  history  as  that  of  the  common 
ferns,  and  is  consequently  attached  to  the  megaspore  (Fig.  282,  Z>). 
But  there  is  an  important  peculiarity  in  its  relation  to  food 
supply.  This  sporophyte  makes  use  of  considerable  food  that 
remains  in  the  megaspore  after  the  development  of  the  female 
gametophyte.  The  Marsilia  plant,  therefore,  actually  provides 
for  the  next  sporophyte  generation  by  storing  food  in  the  mega- 
spore. This  provision  is  strikingly  similar,  as  will  appear  later, 
to  the  conditions  in  the  seed  where  the  embryo  (young  sporo- 
phyte) is  nourished  by  food  stored  in  the  seed  by  the  sporophyte 
of  the  preceding  generation. 

Marsilia  illustrates  exceptionally  well  three  important  prin- 
ciples in  the  evolution  of  pteridophytes  and  seed  plants,  namely : 
(1)  the  establishment  of  heterospory,  resulting  in  the  separation 
of  male  and  female  gametophytes,  (2)  the  reduction  or  degenera- 
tion of  the  gametophytes,  which  become  dependent  upon  food 
stored  in  the  microspores  and  megaspores,  and  (3)  provision  in 
the  megaspore  for  the  nourishment  of  the  embryo  of  the  suc- 
ceeding sporophyte  generation. 


324 


THE  PTERIDOPHYTES 


314.  Heterospory.  Heterospory  (meaning  dissimilar  spores) 
arose  in  the  pteridophytes  with  the  establishment  of  two  sizes 
of  spores,  —  megaspores  and  microspores.  Pteridophytes  having 
these  are  called  heterosporous,  and  those  with  spores  of  the  same 
size,  as  in  the  common  ferns  (Mlicales),  are  called  homosporous 

(meaning  similar  spores).  With 
heterospory  came  also  a  differen- 
tiation of  the  gametophytes  into 
male  and  female  structures,  the 
first  developing  from  the  micro- 
spores  and  the  second  developing 
from  the  megaspores. 

315.  The  moonwort  and 
adders-tongue.  The  moonwort 
(Botrychium,  Fig.  283,  A)  and  the 
adder's-tougue  (Opliioglossum, 
Fig.  283,  B)  are  in  the  same 
group  (order  Opliioglossahs)  and 
illustrate  certain  primitive  con- 
ditions in  the  pteridophytes. 
These  forms  do  not  have  external 
sporangia,  as  in  the  Filicalcs,  but 
the  spores  are  developed  in  sunken 
regions  along  peculiar  stalks. 
Such  sunken  sporangia  are  much 
more  primitive  in  structure  than 
those  which  develop  upon  the  sur- 

A,  the  moonwort  (Botrychium  terna- 

tum) ;  B,  the  adder's-tongue  (Ophi-  face  of  the  plant,for  they  resemble 
ogiossum  vulgatum)  more  ciosely  the  conditions  in  the 

bryophytes  where  the  spore  mother  cells  are  found  in  the  interior 
of  the  plant.  The  spore-bearing  stalks  are  accompanied  by  sterile 
blades  devoted  to  the  vegetative  activities,  so  that  these  leaves 
illustrate  the  same  sort  of  division  of  labor  as  is  found  in  the 
royal  fern  (Osmunda  regalis).  The  gametophytes  are  under- 
ground, tuberous  bodies  generally,  destitute  of  chlorophyll  and 


FIG.  283.    The  moonwort  and 
adder's-tongue 


THE  HORSETAILS  325 

saprophy tic  in  their  life  habits,  being  associated  with  certain  fungi 
which  form  a  mycorrhizal  partnership  (Sec.  278)  with  them. 


CLASS  II.   THE  HORSETAILS,  OR,  SQUISETINE& 

316.  The  horsetails.  The  horsetails,  or  scouring  rushes,  are 
all  comprised  in  the  genus  Equisetum,  which  contains  about  40 
living  species,  the  sole  modern  representatives  of  the  order 
Eguisetales  and  the  class  Equisetinece.  These  plants  are  the 
remnants  of  what  was  a  very  extensive  flora  in  an  early  geo- 
logical period,  called  the  Carboniferous  Age,  when  the  largest 
deposits  of  coal  were  formed.  The  ancient  relatives  of  Equi- 
setum  (Plate  VIII,  2),  together  with  the  club  mosses,  were  then 
trees  and  formed  the  forests  in  those  times.  The  horsetails  live 
now  under  what  seem  to  be  rather  severe  conditions  in  bare  or 
sandy  soils  that  are  unfavorable  for  the  growth  of  trees,  herbs, 
and  grasses.  They  illustrate  very  well  the  way  in  which  an 
ancient  group  is  sometimes  able  to  avoid  total  extinction  by 
withdrawing  as  far  as  possible  from  competition  with  the 
recent  floras,  and  thus  hold  its  own  by  means  of  peculiar  life 
habits  and  adjustments  to  special  conditions. 

The  most  striking  feature  of  the  morphology  of  Eqidsetum  is 
the  total  absence  of  foliage  suitable  for  vegetative  activities 
(photosynthesis).  The  foliage  is  represented  by  sheaths  (Fig.  284, 
Ay  IB),  which  are  found  at  the  joints  of  the  hollow  stem. 
The  points  on  these  sheaths  are  the  tips  of  small  leaves 
more  or  less  united  below.  The  vegetative  functions  are  per- 
formed by  the  green  stems.  These  are  fluted,  that  is,  they  have 
numerous  ridges  which  run  lengthwise,  and  the  depressions 
between  the  ridges  contain  stomata,  which  lie  above  the  chloro- 
phyll-bearing tissues  (Fig.  284,  F,  c).  The  epidermis  contains 
deposits  of  silica,  which  is  the  chief  substance  in  glass,  and  con- 
sequently the  stems  feel  rough.  They  are  sometimes  used  for 
scouring  or  polishing  metal;  hence  the  origin  of  one  of  the 
common  names,  "  scouring  rush." 


FIG.  284.   A  horsetail  (Equisetum  arvense) 

A,  fertile  stems,  bearing  cones  rising  from  the  creeping  rootstock :  t,  tuberous 
bodies;  v,  young  vegetative  stern  below  ground  and  ready  to  grow  into  the 
mature  structure  shown  in  B.  13,  vegetative  stem  as  it  appears  perhaps  three 
weeks  after  the  fertile  stems  have  shed  their  spores  and  died.  C,  a  group  of 
spores  with  their  elaters  expanded.  Z),  a  spore  with  the  elaters  coiled  around 
it.  E,  two  views  of  the  spore  leaves  (sporophylls),  showing  the  group  of  spo- 
rangia. F,  portion  of  a  section  of  the  stem:  a,  air  spaces;  c,  chlorophyll-bear- 
ing tissues ;  r,  rigid  outer  tissues ;  /,  fibro-vascular  bundle  around  small  air 
space.  —  A,  B,  C,  D,  after  Schenck 


326 


THE  CONE  OF  EQUISETUM  327 

The  stems  are  generally  of  two  forms.  There  are  green  aerial 
stems  above  ground,  unbranched  in  some  species,  but  quite 
bushy  in  others  by  the  development  of  circles  of  side  branches 
at  the  joints  (Fig.  284,  B).  The  aerial  stems  arise  from  creeping 
underground  sterns,  or  rootstocks,  which  have  the  same  jointed 
structure  and  sheaths  of  degenerate  leaves,  but  are  not  green 
and  often  not  hollow.  The  underground  stems  live  from  year 
to  year  and  grow  rapidly  through  the  soil,  frequently  estab- 
lishing large  beds  of  horsetails,  as,  for  example,  along  railroad 
tracks  and  the  margins  of  sandy  pools  and  ponds. 

The  stem  has  large,  central  air  cavities,  running  from  joint  to 
joint,  and  also  a  number  of  smaller  air  canals,  alternating  with 
the  fibro-vascular  bundles  (Fig.  284,  F,  a).  It  is  strengthened 
by  thick- walled  cells,  forming  a  rigid  tissue  (Fig.  284,  F,  r)  under 
the  epidermis,  and  is  consequently  well  protected  from  the  dan- 
ger of  drying  up.  These  peculiarities,  together  with  the  reduced 
leaf  surface,  are  characters  which  the  horsetails  have  in  common 
with  many  desert  plants  (xerophytes),  and  they  permit  them  to 
live  when  necessary  under  very  severe  drought  conditions. 

The  fructification  of  Equisetum  is  a  cone  (Fig.  284,  A)  devel- 
oped at  the  tip  of  the  stem,  and  it  is  composed  of  scale-like 
spore  leaves  (sporophylls),  which  fit  closely  together  and  develop 
spores  in  sporangia  upon  their  under  surfaces  (Fig.  284,  E). 
These  cones  are  generally  found  on  ordinary  green  stems.  How- 
ever, in  some  species,  as  E.  arvense,  the  stems  which  first  appear 
above  ground  are  pale  in  color  and  are  devoted  entirely  to  the 
development  of  the  cones  and  die  after  the  spores  are  shed,  while 
the  green  vegetative  stems  appear  later. 

317.  The  cone  of  Equisetum.  A  cone,  or  strobttus,  is  a  com- 
pact group  of  spore  leaves  (sporophylls)  distributed  around  the 
tip  of  a  stem  and  distinct  from  the  rest  of  the  plant.  It  takes 
its  compact  form  because  the  sporophylls  are  closely  set  together 
and  frequently  so  much  modified  that  their  structure  is  not 
apparent  at  a  glance.  Each  sporophyll  in  Equisetum  consists 
of  a  short  stalk  attached  to  the  side  of  the  stem  and  bearing  an 


328 


THE   PTERIDOPHYTES 


angled  shield-shaped  top  (Fig.  284,  E~).  A  group  of  sporangia 
hang  down  all  around  the  stalk  from  the  lower  surface  of  the 
shield,  and  each  develops  from  a  group  of  cells  instead  of  from 
a  single  cell,  as  in  the  common  ferns  (Filicales).  The  shields 
separate  from  one  another  when  the  cone  matures,  and  the  ripe 
spores  escape  through  rents  in  the  sporangia  and  sift  out  between 

the  shields.  The  spores  are 
formed  in  groups  of  four 
(tetrads)  in  the  spore  mother 
cells. 

Each  spore  (Fig.  284,  C,  D) 
bears  four  filaments  de- 
veloped from  an  outer  layer 
of  the  spore  wall,  which 
splits  into  bands  that  sepa- 
rate from  one  another,  but 
remain  attached  to  the  spore 
at  one  point.  The  filaments 
coil  around  it  when  moist, 
but  loosen  and  spread  out 
when  dry.  These  movements 
must  assist  the  escape  of  the 
spores  from  the  sporangia. 
The  filaments  also  serve  as 
wings  in  the  distribution  of 
the  spores  by  the  wind,  and 


FIG.  285.   Gametophytes  of  Equisetum 


-4,  male  prothallium:  an,  antheridium. 
B,  sperms.    C,  female  prothallium :    ar, 

archegonium.-^,  C,  after  Hofmeister;  they  become  entangled  with 

B,  after  Schacht  one  another  so  that  groups 

cling  together  and  are  carried  away  and  germinate  in  clusters. 

The  spores  are  of  the  same  size,  and  therefore  Equisetum  is 

homosporous. 

318.  The  gametophytes  of  Equisetum.  The  spores  only 
retain  their  vitality  for  a  few  days.  They  produce  green  gameto- 
phytes somewhat  like  fern  prothallia,  but  very  irregular  in  form 
(Fig.  285,  A,  C),  the  larger  with  long  lobes,  at  the  bases  of  which 


THE  CLUB  MOSSES  329 

are  situated  the  sexual  organs.  The  prothallia  are  normally  dioe- 
cious, that  is,  male  and  female  in  sex,  but  since  the  spores  are 
distributed  in  groups,  antheridial  plants  are  likely  to  develop  in 
the  same  cluster  with  the  archegonial.  The  sexual  organs  are 
sunken  in  the  tissues  of  the  gametophytes.  The  sperms  are 
coiled,  many-ciliate  protoplasts  (Fig.  285,  B)  resembling  those 
of  the  common  ferns. 

The  early  stages  in  the  development  of  the  young  Equisetum 
sporophyte  from  the  fertilized  egg  are  the  same  as  those  of  the 
common  ferns.  This  together  with  the  similar  gametophytes 
and  sperms  is  believed  to  indicate  a  distant  relationship  between 
the  Equisetinece  and  Filicinece,  even  though  the  mature  sporo- 
phytes  of  the  two  groups  appear  so  different  in  structure. 

CLASS  III.  THE  CLUB  MOSSES,  OB  LYCOPODINE^E 

319.  The  club  mosses.  The  Lycopodinece  take  their  common 
name  of  club  mosses  from  the  moss-like  appearance  of  the  stems, 
which  in  most  forms  are  covered  with  small  leaves  (Figs.  286, 
289,  A),  and  the  fructification,  which  is  generally  a  club-shaped 
cone  developed  at  the  end  of  the  stem  (Figs.  287,  A\  289,  A). 
Isoetes  (Fig.  291)  is,  however,  in  these  particulars  a  conspicuous 
exception.  But  the  club  mosses  are  very  much  larger  than  any 
of  the  true  mosses  (Musci),  and  are  of  course  sporophytes,  like 
the  horsetails  and  ferns,  while  the  true  mosses  are  gameto- 
phytes. Like  the  horsetails,  they  are  the  remnants  of  a  very 
ancient  group  which  formed  forests  in  the  Carboniferous  Age 
(Plate  VIII,  3,  4) ;  also,  they  have  been  able  to  persist  only  by 
adapting  themselves  to  life  conditions  where  they  do  not  en- 
counter keen  competition  with  grasses  and  herbs.  Almost  all 
of  the  Lycopodinece  are  contained  in  three  genera :  Lycopodium 
(about  100  species),  Selaginella  (about  500  species),  and  Isoetes 
(some  60  species).  But  in  addition  there  are  several  remark- 
able types  (Pliylloglossum,  Psilotum,  Tmesipteris)  which  are 
tropical  or  sub-tropical  and  cannot  be  described  here. 


330 


THE  PTERIDOPHYTES 


320.  Lycopodium.  Lycopodium  includes  the  larger  club 
mosses,  frequently  called  lycopods,  and  are  distinguished  by  hav- 
ing needle-like  leaves  arranged  spirally  on  the  stem  (Fig.  286) 
and  similar  spores  (homosporous).  The  stems  are  of  two  forms : 
(1)  creeping  stems,  close  to  the  ground  and  frequently  buried 


FIG.  286.   A  club  moss  (Lycopodium  annotinum) 
Modified  after  Kerner 

under  leaves  and  other  forest  debris,  and  (2)  upright  sterns,  very 
much  branched  in  some  species  and  bearing  the  cones  like 
clubs  at  their  ends.  Some  of  the  larger  species  are  very  common 
in  the  northern  woods,  the  long,  creeping  stems  often  growing 
thickly  over  the  ground.  The  stem  is  generally  quite  woody  in 
structure,  and  the  leaves  are  evergreen.  They  are  much  used 
in  holiday  decoration,  and  certain  species  are  in  danger  of  extinc- 
tion, since  the  club  mosses  reproduce  very  slowly. 


THE  CONE  OF  LYCOPODIUM 


331 


321.  The  cone  of  Lycopodium.    In  some  species  of 

as  L.  Selago,  the  spore  leaves  (sporophylls)  have  the 
same  form  and  grouping  as  the  vegetative  leaves  so  that  there 
is  no  cone  distinct  from  the  rest  of  the  stem.  But  most  of  the 
forms  have  very  clearly  defined  cones,  which  are  sometimes  raised 
on  long  stalks,  as  in  L.  compla- 
natum.  The  sporophylls  are 
generally  scale-like  and  closely 
set  (Fig.  287,  A,  B).  Each  spore 
leaf  bears  a  single,  large,  sac-like 
sporangium  (Fig.  287,  C)  at  its 
base,  which  develops  from  a 
group  of  cells.  The  spores  (Fig. 
287,  D)  are  formed  in  groups 
of  four  (tetrads)  in  the  spore 
mother  cells.  They  are  very 
minute  and  are  produced  in  such 
immense  numbers  that  they  are 
collected  in  quantity  as  the 
lycopodium  powder  of  apothe- 
cary shops,  used  in  dusting 
pills  to  keep  them  from  sticking 
together  as  well  as  for  other 
purposes.  This  powder  is  also 
employed  in  the  manufacture 
of  fireworks  under  the  name  of 
vegetable  sulphur. 

322.  The  gametophytes  of 
Lycopodium.  The  gametophytes 

of  the  club  mosses  in  our  northern  woods  must  be  uncommon,  if 
they  are  developed  at  all,  for  they  have  never  been  found.  It  is 
probable  that  the  sporophy  tes  reproduce  chiefly  or  perhaps  entirely 
by  vegetative  brandling  of  their  stems  and  in  some  forms  by 
curious  buds.  The  gametophytes  of  some  tropical  lycopods  are 
however  known  and  have  been  studied.  They  are  small,  tuberous 


FIG.  287.   The  cone  of  a  club  moss 
(Lycopodium  annotinum) 

4,  the  cone,  showing  overlapping  sporo- 
phylls; i>,  diagram  of  a  longitudinal 
section,  illustrating  the  form  and 
position  of  the  sporophylls  and  spo- 
rangia ;  C,  the  inner  face  of  a  sporo- 
phyll,  showing  the  large  sporangium ; 
.D,  two  views  of  spores  from  a  group 
of  four  (tetrad) 


332 


THE  PTERIDOPHYTES 


bodies  (Fig.  288)  generally 


FIG.  288.  Gametophytes  and 
young  sporophy  tes  of  a  club  moss 
(Lycopodium  complanatum) 

A,  gametophyte  with  young  sporo- 
phyte :  /,  tissue  filled  with  the  fila- 
ments of  a  fungus  situated  just 
outside  a  layer  of  palisade  cells. 

B,  the  fungus-infected  tissue.    C, 
a  young  club  moss  still  attached 
to  the  subterranean  gametophyte. 
—  A,  B,  from  material  of  Bruch- 
mann    prepared   by    Miss  Lyon ; 

C,  after  Bruchmann 


subterranean  and  practically  desti- 
tute of  chlorophyll,  like  those  of 
the  moonwort  and  adder's-tongue. 
They  are  therefore  saprophytic,and 
associated  with  them  are  fungal 
filaments  to  form  a  mycorrhizal 
relation  (Sec.  278).  The  sexual 
organs  are  sunken  structures. 
The  sperms  are  two-ciliate. 

The  embryo  sporophyte  remains 
attached  to  the  gametophyte  by  a 
large  foot  (Fig.  288)  for  a  long 
time  after  the  stem  and  root  are 
developed,  and  must  obtain  much 
nourishment  from  the  gameto- 
phyte, as  in  the  case  of  the  ferns. 

323.  Selaginella.  Sclaginclla 
is  readily  distinguished  from  Lyco- 
podium. The  leaves  in  most 
species  are  arranged  in  four  rows, 
two  rows  of  smaller  leaves  on  the 
upper  surface  and  two  rows  of 
larger  leaves  somewhat  at  the  sides 
(Fig.  289,  A).  The  cones  also  have 
four  rows  of  spore  leaves  (sporo- 
phylls)  and  are  consequently  four- 
angled.  The  spores  are  of  two 
sizes,  and  the  type  is  perhaps  the 
best  illustration  of  heterospory  in 
the  pteridophytes.  Forms  of  Sel- 
aginella are  frequently  called 
"  little  club  mosses,"  for  many  of 
them  are  much  more  delicate  than 
the  lycopods.  But  some  tropical 
species,  frequently  cultivated  in 


SELAGINELLA 


333 


greenhouses,  are  large,  much-branched,  and  bushy  plants,  very 
graceful  and  decorative.  Some  forms  grow  in  dry  situations 
on  sand  and  rocks,  in  Mexico  and  the  Southwest.  One  species 
(S.  lepidopJiylla)  is  frequently  sold  in  the  North  under  the  name 


FIG.  289.   Selaginella  Martensii 

A,  branch  bearing  cones  and  showing  the  leaf  arrangements;  B,  inner  face  of 
a  megasporophyll,  showing  the  large  megasporangium  containing  a  group 
of  four  megaspores  (tetrad);  C,  two  views  of  megaspores;  D,  inner  face  of 
microsporophyll,  showing  microsporangium ;  E,  microspores ;  F,  diagram  of 
a  longitudinal  section  of  cone  illustrating  position  of  microsporophylls  and 
megasporophylls  and  their  microsporangia  and  megasporangia 

of  "  resurrection  moss."  This  plant  protects  itself  during  drought 
by  rolling  up  the  branches  to  form  a  compact  ball.  When 
moistened  the  branches  spread  out  and  become  fresh  and  green.1 

1  A  botanist  states  that  the  plants  sold  in  the  North  will  absorb  moisture 
and  unroll,  but  are  generally  "  dead  "  beyond  recovery. 


334  THE  PTERIDOPHYTES 

324.  The  cones  of  Selaginella.    The  coues  of  Selaginella  are 
not  as  large  as  those  of  Lycopodium,  but  they  are  much  more 
complex  in   structure.    The   sporangia  are  of  two  sorts,  both 
developing  singly  from  a  group  of  cells  on  the  stem  just  above 
the  origin  of  the  spore  leaves  and  later  becoming  attached  to 
their  bases.    The   sporangia  near  the  lower  part  of  the  cone 
(Fig.  289,  B)  produce  from  one  to  eight  very  large  megaspores, 
and  frequently  a  group  of  four  (tetrad).    The  sporangia  higher 
up  on  the  cones  (Fig.  289,  />)  are  smaller  and  develop  a  great 
number  of  minute  microspores,  also  in  tetrads.   Selaginella  has, 
then,  different  sporangia  for  the  two  forms  of  spores,  microspores 
and  megaspores,  which  are  accordingly  called  microsporangia 
and   megasporangia.    Furthermore,  these   sporangia   are   borne 
upon   different    spore  leaves,  which   are   consequently  termed 
microsporopliylls  and  megasporopliylls.    It  is  important  to  note 
that  the  few  megaspores  which  mature,  are  nourished  and  grow 
at  the  expense  of  neighboring  spore  mother  cells  which  become 
disorganized. 

325.  The   gametophytes    of    Selaginella.    The    microspore 
develops   a  reduced  and  degenerate  male  gametophyte,  as  in 
Marsilia  (Sec.  313).    There  is  a  small  sterile  cell  (prothallial 
cell)  and  two  groups  of  sperm  cells  in  a  very  simple  cellular 
structure  probably  representing  an  antheridium  (Fig.  290,  A). 
The  sperms  are  two-ciliate  (Fig.  290,  B). 

The  megaspore  develops  a  female  gametophyte  which  is  larger 
than  that  of  Marsilia,  but  it  is  the  same  sort  of  a  reduced  struc- 
ture, dependent  upon  food  stored  by  the  sporophyte  within  the 
megaspore.  This  gametophyte  at  maturity  fills  the  megaspore, 
and  bursting  through  the  spore  wall  it  presents  an  exposed  sur- 
face upon  which  several  sunken  archegonia  are  developed  (Fig. 
290,  C).  The  female  gametophyte  actually  begins  its  develop- 
ment before  the  megaspore  has  attained  its  full  size  in  the  mega- 
sporangium.  It  is  thus  parasitic  upon  the  sporophyte  during  its 
early  history,  a  habit  which  is  universal  in  the  seed  plants,  but 
among  the  pteridophytes  it  is  only  found  in  Selaginella. 


THE  GAMETOPHYTES  OF  SELAGINELLA 


335 


There  are  some  other  life  habits  of  Sclaginella  wonderfully 
suggestive  of  the  way  in  which  seed  and  the  seed  habit  arose. 
It  is  known  in  some  species,  as  S.  rupestris,  that  the  micro- 
spores  are  thrown  out  from  the  sporangia  on  the  upper  part  of 
the  cone  and  sift  down  like  pollen  grains  among  the  megaspores 


FIG.  290.   The  gametophytes  and  embryo  of  Selaginella 

A,  male  gametophyte  contained  within  the  microspore :  p,  persistent  nucleus  of 
prothallial  cell ;  s,  two  groups  of  sperm  mother  cells.    B,  two-eiliate  sperms. 

C,  female  gametophyte  containing  an  emhryo  sporophyte :  a,  archegonium  ; 
r,  rhizoids.    I),  young  sporophytes  held  by  the  spore  leaves  of  the  cone.    E,  a 
young  sporophyte  still  attached  to  the  megaspore.  — J5,  after  Belajeff ;  A,  C, 

D,  adapted  after  notes  and  sketches  of  Miss  Lyon 


336  THE  PTERIDOPHYTES 

into  the  split  megasporangia  below.  The  sperms  are  formed 
and  set  free  in  the  moisture  of  such  situations,  and  the  eggs  of 
the  gametophytes  may  be  fertilized  while  the  megaspores  are 
still  retained  within  the  megasporangium.  The  young  sporo- 
phytes  as  they  develop  are  thus  actually  held  by  the  sporophylls 
of  the  parent  sporophyte  (Fig.  290,  Z>)  until  they  reach  a  con- 
siderable size  and  fall  off.  These  habits  should  be  noted  and 
this  paragraph  read  again  after  the  life  history  of  the  seed  plant 
is  thoroughly  understood. 

The  development  of  the  young  sporophytes  of  Selaginella 
and  also  of  Lycopodium  has  features  resembling  those  of  the 
seed  plants.  The  early  divisions  of  the  egg  establish  a  structure 
called  the  suspensor  (Fig.  290,  (7,  Sus),  which  carries  the  devel- 
.  oping  embryo  down  into  the  midst  of  the  tissue  of  the  gameto- 
phyte,  where  it  can  draw  nourishment  from  all  of  the  cells 
around  it.  A  large  foot  is  developed  (Fig.  290)  which  absorbs 
food  from  that  portion  of  the  gametophyte  which  lies  in  the 
megaspore,  so  that  the  embryo  sporophyte  is  actually  nourished 
with  food  stored  in  the  megaspore  by  the  sporophyte  of  the 
previous  generation. 

326.  Life  history  of  Selaginella.  Selaginella  is  an  excellent 
type  with  which  to  illustrate  the  life  history  of  a  heterosporous 
pteridophyte.  Since  two  forms  of  spores,  microspores  and  mega- 
spores,  are  present,  there  are  two  forms  of  gametophytes,  male 
and  female,  and  this  feature  complicates  the  relatively  simple 
life-history  formulae  of  bryophytes  and  homosporous  pterido- 
phytes  (Sees.  285,  310). 

The  life  history  of  Selaginella  is  as  follows  : 


~          ,       ^microspore  —  Male  Gametophyte  —  sperm  . 
P      P  y     ^megaspore  —  Female  Gametophyte  —  egg   - 

—  Sporophyte,  etc. 
This  in  abbreviated  form  becomes 

mi  sp  —  M  G  —  s  .         0 


SUMMARY  OF  SELAGIKELLA  337 

which  when  carefully  studied  is  essentially  the  same  as  the 
general  life-history  formula  of  bryophytes  and  pteridophytes 
(Sees.  285,  310),  namely, 

-S-sp-G,  etc. 

The  differences  lie  in  the  fact,  above  mentioned,  that  there  are 
two  forms  of  spores,  microspores  and  megaspores,  which  develop, 
respectively,  male  and  female  gametophy tes,  a  complication  which 
was  introduced  with  heterospory  and  which  is  present,  as  will 
appear  later,  in  the  life  histories  of  seed  plants  (Sec.  356). 

327.  Summary  of  Selaginella.  Selaginella  is  the  highest  of  the 
pteridophytes  and  the  most  important  because  of  the  evolution- 
ary principles  which  it  illustrates,  leading  up  to  the  seed  habit. 
The  first  three  of  these  principles  are  also  illustrated  by  Mar- 
silia  and  Isoetes,  but  the  fourth  and  fifth  are  new.    They  are 
(1)  the  establishment  of  heterospory  resulting  in  the  separation 
of  male  and  female  gametophytes ;  (2)  the  reduction  or  degen- 
eration of  the  gametophyte,  which   becomes   dependent  upon 
food  stored  in  the  microspores  and  megaspores ;  (3)  provision 
in  the  megaspore  for  the  nourishment  of  the  embryo  of  the  suc- 
ceeding sporophyte  generation;  (4)  the  early  parasitic  relation 
of  the  female  gametophyte  within  the  megasporangium  upon  the 
sporophyte;  (5)  the  occasional  habit  of  developing  the  young 
sporophyte  while  the    megaspore  is   still  retained  within  its 
parent  megasporangium. 

328.  .Isoetes.    The  species  of  Isoetes  are  known  as  quillworts. 
Their  position  among  the  pteridophytes  is  a  matter  of  dispute, 
and  some  botanists  place  them  with  the  ferns  (Filicmece),  but 
the  anatomy  of  the  sporophytes  is  more  like  that  of  the  club 
mosses  than  any  other  group.    They  have  a  peculiar  rush-like 
habit  of  growth,  the  long  leaves  arising  in  clusters  around  a 
short  stem  (Fig.  291,^4).    Some  forms  are  aquatic,  growing  on 
mud  at  the  bottom  of  ponds,  while  others  are  usually  found  out 
of  water. 


338  THE  PTERIDOPHYTES 

Isoetes  is  heterosporous,  and  the  spores  are  developed  in  sunken 
sporangia  at  the  bases  of  spore  leaves  (Fig.  291,  B,  s).  The  spore 
leaves  are  differentiated  so  that  only  the  outermost  develop 
megaspores  and  are  consequently  megasporophylls,  while  the 


FIG.  291.   The  quill  wort  (Isoetes  echinospora) 

A,  habit  sketch.  B,  base  of  megasporophyll,  showing  inner  surface :  s,  sporangium, 
containing  the  large  megaspores ;  Z,  ligule.  C,  a  group  of  microspores  below, 
and  a  large  megaspore  above,  showing  comparative  size 

innermost  are  microsporophylls,  producing  only  microspores. 
Male  and  female  gametophytes  are  developed  slowly  in  the 
microspores  and  megaspores,  respectively,  and  are  reduced  or 
degenerate  sexual  plants  (Fig.  292,  A,  C),  almost  as  simple  as 


FOSSIL  PLANTS 


339 


those  of  Marsilia.  Having  no  chlorophyll,  they  depend  upon  food 
stored  in  the  megaspore,  as  in  Marsilia  and  Selayinella.  The 
young  sporophyte  also  makes  use  of  food  in  the  megaspore  as 
in  these  other  two  heterosporous  pteridophytes  above  mentioned. 
The  sperms  (Fig.  292,  B)  are  some- 
what coiled  and  many-ciliate,  re- 
sembling in  this  respect  those  of 
the  Filicineoe.  The  life- history 
formula  is  the  same  as  that  of  Sel- 
aginella  (Sec.  326). 

FOSSIL  PLANTS  AND  COAL 

329.  Fossil  plants.  Plant  re- 
mains are  not  generally  preserved 
as  fossils,  partly  because  they  do 
not  often  have  hard  parts,  such  as 
the  shells  and  bones  of  animals, 
and  partly  because  the  larger  forms 
grow  on  land  where  they  are  sub- 
ject to  rapid  decay.  So  the  record 
of  plant  life  in  former  geological 


FIG.  292.   Gametophytes  of  the 
quillwort  (Isoetes) 


sperm  mother  cells  shown  within 
the  reduced  antheridium.  Z>, 
sperm.  (7,  section  of  female  game- 
tophyte  removed  from  megaspore, 
showing  sunken  archegonium.  A, 
C,  Isoetes  echinospora. — A,  C\ 
after  Campbell;  /?,  after  Belajeff 


,        . ,  ,      A,  male  gametophyte  within  the  mi- 
ages   IS    poor   as    Compared  With       crospore:  p.prothallialcell;  four 

that  of  animal  life.  However,  there 
are  some  very  wonderful  deposits 
of  plant  remains  forming  the  hard 
and  soft  coal  beds,  which  de- 
serve brief  mention  here,  since 
most  of  the  plants  composing  them  are  fossil  pteridophytes. 
During  the  Devonian  and  Carboniferous  Ages  the  most  con- 
spicuous vegetation  was  represented  by  tree  ferns  and  relatives 
of  the  horsetails  and  club  mosses,  together  with  certain  very 
primitive  gymnosperms.  These  plants  reached  the  height  of 
trees  and  formed  forests  on  the  land  and  in  the  marshes  (see 
Plate  VIII).  The  Catamites  (Plate  VIII,  2)  were  gigantic 


340  THE  PTERIDOPHYTES 

horsetails,  so  nearly  like  the  living  forms  of  Equisctum  that  we 
can  readily  picture  their  appearance  along  the  margins  of  swamps 
and  streams.  Curiously  some  of  the  Catamites  were  heterospor- 
ous,  although  all  of  the  living  types  of  the  Equisctineoe  are 
homosporous.  The  ancient  representatives  of  the  club  mosses 
(Plate  VIII,  3,  4)  were  among  the  largest  plants  of  those  times, 
reaching  the  height  of  one  hundred  feet  or  more.  Some  of  them 
were  true  lycopods,  and  others,  as  Lepidodendron  and  Sigillaria, 
were  evidently  close  relatives  of  the  club  mosses.  Their  large 
trunks  were  covered  with  leaves,  which  fell  off,  leaving  curious, 
diamond-shaped  scars  that  are  very  conspicuous  on  the  fossil 
stems.  The  earliest  seed  plants  arose  in  these  ages,  but  they 
were  far  outnumbered  by  the  pteridophytes.  They  were  gymno- 
sperms  of  the  group  Cordaitece  (Plate  VIII,  5),  but  with  very 
little  resemblance  to  any  living  forms.  The  fructifications  of 
some  of  these  primitive  forms,  somewhat  intermediate  between 
spermatophytes  and  pteridophytes,  are  occasionally  so  well  pre- 
served that  we  can  learn  something  of  the  structure  of  the 
gametophytes  developed  by  the  spores.  It  is  possible  that  we 
shall  later  know  much  more  about  the  origin  of  the  seed  plants 
and  the  seed  habit  from  the  study  of  these  fossils. 

After  the  Carboniferous  Age  the  tree  ferns,  horsetails,  and 
club  mosses  became  less  abundant,  and  gymnosperms,  like  the 
cycads  and  conifers,  increased  in  numbers  and  became  the 
dominant  forest  types.  There  was  an  age  of  cycads  in  a  later 
period  (Jurassic),  when  the  earth  was  covered  with  these  plants 
as  far  north  as  Greenland  and  the  climate  must  have  been 
tropical  from  pole  to  pole.  We  know  very  little  about  the 
earliest  forms  of  angiosperms.  They  do  not  appear  abundantly 
as  fossils  until  a  later  period  (Cretaceous),  after  the  age  of 
cycads  (Jurassic),  although  they  doubtless  had  their  origin 
much  earlier,  for  many  insects  were  present  which  must  have 
had  the  habit  of  feeding  on  pollen  or  nectar. 

It  is  clear  that  the  horsetails  and  club  mosses  of  the  present 
time  are  merely  the  remnants  of  this  ancient  flora  once  dominant 


COAL  341 

and  perhaps  as  luxuriant  as  the  tropical  forests  of  to-day.  They 
have  survived  by  adjusting  themselves  to  very  different  life 
conditions  from  those  of  Carboniferous  times,  and  by  adopting 
life  habits  which  remove  them  as  far  as  possible  from  competi- 
tion with  the  prevailing  vegetation  forms  of  to-day  (trees,  grasses, 
herbs,  etc.).  The  degenerate,  saprophytic  gametophytes  of  Ly co- 
podium  illustrate  well  how  far  such  changes  of  life  habits  may 
extend. 

330.  Coal.  Coal  was  formed  during  a  number  of  periods  in 
the  earth's  history,  but  the  most  extensive  deposits  were  laid 
down  during  the  Carboniferous  Age  (frequently  called  the  coal 
age),  forming  the  so-called  coal  measures.  The  luxuriant  pterido- 
phyte  vegetation  of  tree  ferns,  horsetails,  and  club  mosses  formed 
deposits  in  swamps  over  immense  areas,  probably  in  much  the 
same  way  as  peat  is  being  formed  to-day.  Such  plant  deposits 
from  time  to  time  became  covered  with  sediment  by  the  sink- 
ing of  the  land.  And  since  the  land  alternately  rose  and  sank, 
successive  layers  or  beds  of  plant  remains  were  laid  down. 
These  remains  became  finally  buried  under  heavy  deposits  of 
sediment,  which  pressed  them  into  compact  beds  of  the  car- 
bonaceous matter  called  coal. 

Coal  is  of  two  sorts:  (1)  soft  or  bituminous  coal,  which  is 
hardly  more  than  half  pure  carbon,  the  rest  being  composed  of 
a  variety  of  carbon  compounds,  and  (2)  hard  or  anthracite  coal, 
which  may  be  90  per  cent  pure  carbon.  Hard  coal  represents  a 
greater  degree  of  change  than  soft  coal,  the  oils  and  other  products 
having  been  driven  off  under  pressure  by  the  heat  of  the  earth. 
The  coal  beds  vary  in  thickness  from  small  layers  of  only  a  few 
inches  to  deposits  a  hundred  feet  deep.  Those  of  the  United 
States  cover  several  hundred  thousand  square  miles,  of  which 
perhaps  fifty  thousand  square  miles  are  being  worked.  Vast  as 
are  these  coal  beds  in  the  United  States,  there  are  deposits  in 
other  lands,  as  in  China,  of  even  greater  extent.  The  coal  supply 
of  China  is  estimated  as  enough  to  last  the  world  seven  hundred 
years.  The  total  deposits  of  pteridophyte  vegetation  were  very 


342  THE  PTERIDOPHYTES 

much  thicker  than  the  coal  beds  which  they  formed,  for  it  has 
been  estimated  that  it  took  about  five  feet  of  plant  remains  to 
make  one  foot  of  coal. 

It  is  interesting  to  think  of  the  part  which  the  pteridophyte 
flora  of  the  Carboniferous  Age  plays  in  the  present  life  and 
economic  activities  of  the  world,  giving  us  a  fuel  whose  carbon 
was  taken  ages  ago  from  the  air,  which  was  then  much  more 
heavily  charged  with  carbon  dioxide  than  is  the  atmosphere  of 
to-day. 

THE  ORIGIN  AND  EVOLUTION  OF  THE 
PTERIDOPHYTES 

331.  The  origin  of  the  pteridophytes.  The  pteridophytes 
undoubtedly  arose  from  a  bryophyte  ancestry,  when  the  sporo- 
phyte  generation,  in  some  forms  having  a  structure  capable  of 
doing  chlorophyll  work,  developed  a  root  system  and  vascular 
tissues,  and  taking  the  land  habit  became  independent  of  the 
gametopliyte.  This  was  one  of  the  most  important  forward  steps 
in  the  evolution  of  the  higher  plants,  for  it  gave  the  sporophyte 
complete  freedom  to  live  and  grow  to  its  maximum  size.  It 
marked  a  turning  point  in  plant  evolution,  for  after  that  the 
sporophyte  became  the  most  complex  and  conspicuous  phase  of 
the  life  history,  and  the  gametopliyte  grew  less  prominent,  until 
finally  in  the  seed  plants  the  sexual  generation  becomes  actually 
dependent  or  parasitic  upon  the  asexual  generation,  a  relation- 
ship which  is  exactly  the  reverse  of  that  between  the  gameto- 
phyte  and  sporophyte  in  the  liverworts  and  mosses.  These  very 
important  results  in  the  evolution  of  plants  are  summarized  in 
Chapter  xx IX,  The  Evolution  of  the  Sporophyte  and  Degenera- 
tion of  the  Gametophyte. 

There  are  no  bryophytes  that  show  clearly  how  the  root 
system  arose,  but  we  can  easily  understand  that  so  complex  a 
sporophyte  as  that  of  Anthoceros  (which  has  chlorophyll-bearing 
tissues  with  stomata,  and  a  long,  indefinite  period  of  growth) 
would  at  once  become  an  independent  plant,  if  it  could  develop 


THE  EVOLUTION  OF  THE  PTERIDOPHYTES   343 

a  root  system.  For  this  reason,  Anthoceros  (Sec.  290)  is  gen- 
erally considered  the  form  among  the  bryophytes  most  closely 
approaching  the  pteridophytes  in  its  structure  and  possibilities 
«pf  development. 

332.  The  evolution  of  the  pteridophytes.   After  the  sporophyte 
became  independent  of  the  gametophyte,  the  next  important 
advance  was  the  development  of  the  lateral  spore-bearing  and 
vegetative  organs  called  fronds.    Then  came  the  differentiation 
of  the  fronds  into  vegetative  leaves,  given  up  entirely  to  chloro- 
phyll work  (photosynthesis),  and  spore  leaves,  or  sporophylls, 
devoted  chiefly  or  wholly  to  spore  production.    With  this  also 
came  the  massing  of  the  sporophylls  in  cones,  which  was  really 
the  beginning  of  the  structures  called  flowers  in  the  seed  plants. 

Finally,  the  condition  of  heterospory  was  attained  independ- 
ently in  several  groups  of  the  pteridophytes,  as  the  water  ferns, 
Selaginella,  and  Isoetes.  Heterospory  soon  led  to  very  signifi- 
cant changes  in  the  structure  and  behavior  of  the  gametophyte 
generations.  They  became  differentiated  in  sex,  the  microspores 
producing  male  prothalli,  and  the  megaspores  female  ones.  Fur- 
thermore, the  gametophytes  became  greatly  reduced,  finally  de- 
pending wholly,  or  almost  wholly,  on  food  stored  in  the  spores. 
The  food  in  the  female  gametophyte  also  came  to  contribute  to 
the  development  of  the  embryo  sporophyte,  which  was  thus  fur- 
nished with  food  by  the  sporophyte  of  the  previous  generation. 
At  last,  in  the  highest  form,  Selaginella,  the  female  gametophyte, 
begins  its  development  while  still  retained  within  the  megaspore, 
a  condition  approximating  very  closely  to  the  seed  habit. 

SUMMARY  OF  THE  PTERIDOPHYTES  AND  THEIR 
ADVANCES  OVER  THE  BRYOPHYTES 

333.  Summary  of  the  pteridophytes.    The  chief  characters 
of  the  pteridophytes  and  their  advances  over  the  bryophytes  are : 

1.  The  development  of  a  leafy  shoot  and  root  system  with 
vascular  tissues  in  the  sporophyte  generation,  rendering  it 


344  THE   PTERIDOPHYTES 

independent  of  the  gametophyte,  giving  it  the  land  habit,  allow- 
ing it  to  attain  a  large  size,  and  making  it  by  far  the  most 
conspicuous  phase  in  the  life  history. 

2.  The  development  and  differentiation  of  fronds  into  vege- 
tative leaves  and  sporophylls,  and  the  grouping  of  the  latter 
into  cones. 

3.  The  development  of  heterospory,  which  differentiated  the 
gametophytes  as  male  and  female  in  sex. 

4.  The   degeneration  of  the  gametophytes  (in  heterosporous 
forms)  so  that  they  finally  became  dependent  upon  food  supplied 
by  the    sporophyte  in   the  spore.    In   Selaginella   the   female 
gametophyte  even  begins  its   development  at  the  expense  of 
neighboring  cells  in  the  megasporangium.    These  conditions  are 
an  exact  reversal  of  the  relations  between  the  generations  which 
exist  in  the  bryophytes. 

The  three  classes  of  the  Pteridophyta  are  readily  distin- 
guished by  the  f ollowing,  characters  : 

Class  I.  Filicinece.  Fronds  large  and  few  in  number ;  those 
bearing  spores  generally  similar  to  the  strictly  vegetative  leaves 
and  not  grouped  in  cones. 

Class  II.  Equisetinece.  Leaves  reduced  to  mere  scales,  form- 
ing sheaths  around  jointed  stems,  which  have  many  peculiarities 
of  structure ;  sporophylls  of  peculiar  form,  each  bearing  several 
sporangia,  and  grouped  in  a  characteristic  cone. 

Class  III.  Lycopodinece.  Vegetative  leaves,  small  and  very 
numerous  (except  in  Isoetes},  covering  the  stem ;  sporophylls 
generally  grouped  in  cones,  each  bearing  a  single  sporangium ; 
gametophytes  much  degenerate,  especially  in  the  heterosporous 
Selaginella  and  Isoetes;  sperms  two-ciliate,  except  in  Isoetes, 
and  not  spiral,  and  many-ciliate  as  in  the  Filicinece  and  Equise- 
tinece. 


CHAPTER  XXVI 
ALTERNATION  OF  GENERATIONS 

334.  The  protoplasmic  basis  of  an  alternation  of  genera- 
tions.* The  history  of  the  alternation  of  generations  in  plants 
has  now  been  traced  from  the  relatively  simple  beginnings  in  the 
thallophytes,  as  illustrated  by  the  life  histories  of  the  red  algre 
(Sec.  246)  and  sac  fungi  (Sec.  26,6)  through  the  more  clearly 
denned  conditions  in  the  liverworts  and  mosses,  and  also  through 
the  ferns,  horsetails,  and  club  mosses.  It  is  clear  that  in  the 
latter  groups  and  the  pteridophytes  the  asexual,  or  sporophyte, 
generation  had  become  much  the  more  complex  of  the  two,  and 
that  the  sexual  generation,  or  gametophyte,  had  begun  to  degen- 
erate. This  degeneration  is  carried  much  further  in  the  seed 
plants,  as  will  be  described  in  Chapter  xxvm,  and  summarized 
in  Chapter  XX ix. 

It  is  now  time  to  try  to  determine  some  of  the  reasons  for 
the  establishment  of  a  sporophyte  generation  following  the  game- 
tophyte one,  or  the  basis  in  the  protoplasm  itself  of  the  alternation 
of  sexual  and  asexual  generations.  The  basis  undoubtedly  rests 
on  the  effects  of  the  sexual  process  upon  the  nature  of  the  pro- 
toplasm in  the  succeeding  generation.  The  union  of  gametes  is 
so  great  a  physiological  stimulus  that  the  sexually  formed  cell 
(generally  a  fertilized  egg)  is  given  the  possibilities  of  a  develop- 
ment different  from  that  of  either  parent  plant  or  gametophyte ; 
for  the  protoplasm  of  a  fertilized  egg  is  not  the  same  as  that  of 
either  gamete  which  entered  into  its  formation.  It  is  a  mixture 
of  protoplasms  and  therefore  must  be  different  from  the  proto- 
plasm of  the  parent  plants,  and  this  difference  is  the  basis  for 

*  To  THE  INSTRUCTOR  :  In  a  brief  course  or  with  immature  students  this 
chapter  should  be  omitted. 

345 


346       •  ALTERNATION  OF   GENERATIONS 

the  peculiarities  of  the  generation  which  arises  from  a  sexually 
formed  cell 

Protoplasm  has  so  far  proved  much  too  complex  for  an  analy- 
sis into  the  structures  which  determine  its  qualities  and  possi- 
bilities of  development;  that  is,  we  do  not  know  why  the  egg 
of  a  fern  develops  into  a  fern  and  that  of  a  club  moss  into  a 
club  moss;  both  are  cells  with  a  general  similarity  of  cell  struc- 
ture. But  the  possibilities  of  fern  and  club  moss  are  nevertheless 
present  in  the  respective  eggs,  and  the  one  could  not  possibly  be 
made  to  produce  the  other  plant.  It  is  generally  believed  that 
the  characteristics  of  eggs  are  determined  by  the  structure  of 
their  protoplasm,  represented  perhaps  by  means  of  the  invisible 
molecules  and  groups  of  molecules  in  its  chemical  and  physical 
composition.  The  structures  that  are  assumed  to  give  distinct 
character  or  possibilities  of  development  to  protoplasm  are  called 
rudiments. 

It  is  doubtful  whether  we  shall  ever  be  able  to  distinguish 
the  rudiments,  but  there  are  some  larger  structures  in  the  cell 
which  with  care  can  be  followed  through  the  cell  divisions  from 
generation  to  generation.  The  most  interesting  of  these  are 
the  chromosomes,  which  are  very  characteristic  structures  most 
clearly  seen  during  the  processes  of  nuclear  division  (Sec. 
199).  The  substance  of  the  chromosomes,  called  chromatin, 
is  the  most  important  material  in  the  nucleus.  Chromatin  can 
be  deeply  colored  or  stained  in  thin  sections  of  tissue  after 
special  methods  of  treatment.  It  is  present  in  the  resting 
nucleus,  generally  in  the  form  of  an  irregular  network.  The 
chromosomes  are  formed  from  the  chromatin  and  appear  during 
the  early  stages  of  nuclear  division.  Each  chromosome  then 
divides  into  halves,  and  the  two  sets  of  daughter  chromosomes 
are  distributed  with  each  nuclear  division. 

It  is  an  important  fact  that  the  number  of  chromosomes  for 
the  nuclei  of  each  plant  is  fixed,  and  the  number  is  usually 
not  very  large.  Thus  the  gametophytes  of  a  red  alga  (Poly- 
siphonia,  Sec.  245)  have  about  20  chromosomes,  but  those  of 


BASIS  OF  ALTERNATION  OF  GENERATIONS        347 

the  liverwort  (Anthoceros,  Sec.  290)  have  only  4  and  the  fern 
(Osmunda)  12.  The  most  important  feature  of  the  process  of 
fertilization  is  the  union  of  the  two  gamete  nuclei,  that  of  the 
sperm  with  that  of  the  egg.  These  nuclei  have  an  equal  number 
of  chromosomes  in  the  same  species  (the  number  characteristic 
of  the  gametophyte),  and  the  egg  and  sperm  are  therefore  equiv- 
alent in  their  nuclear  structure,  whatever  may  be  the  differences 
in  their  size.  This  nuclear  fusion  doubles  the  number  of  chromo- 
somes, and  the  fertilized  egg  begins  the  development  of  the 
sporophyte  (when  present)  with  twice  as  many  chromosomes  as 
the  gametophytes  which  produced  the  eggs  and  sperms. 

The  double  number  of  chromosomes  appears  in  all  of  the 
nuclear  divisions  throughout  the  development  of  the  sporo- 
phyte up  to  the  time  of  spore  formation.  Thus  the  sporophyte 
phases  of  Polysiphonia  have  about  40  chromosomes,  the  sporo- 
phyte of  Anthoceros  8,  and  Osmunda  24.  The  lilies  have 
24  chromosomes,  and  the  gametophyte  phase  only  12.  The 
chromosomes  have  been  counted  in  more  than  fifty  different 
kinds  of  plants,  mostly  seed  plants,  and  it  is  established  that 
sporophytes  have  normally  double  the  number  of  chromosomes 
of  their  respective  gametophytes. 

Spore  formation  at  the  end  of  the  sporophyte  generation  is  a 
very  significant  period  in  the  life  history,  for  at  this  time  the 
double  number  of  chromosomes  is  reduced  by  half.  The  spores 
have  then  the  original  number  of  the  gametophyte.  The  reduc- 
tion of  the  chromosomes  is  effected  by  processes  too  complicated 
to  be  described  here,  but  the  formation  of  the  asexual  spores  in 
groups  of  four,  called  tetrads  (see  Figs.  204,  245,  258,  289,  298, 
302,  304),  is  rather  characteristic  of  the  phenomenon.  There  are 
thus  fundamental  reasons  for  the  identical  methods  of  spore  for- 
mation in  the  bryophytes  and  pteridophytes,  and,  as  will  appear 
later,  for  the  methods  of  pollen  formation  and  the  embryo  sac 
in  the  seed  plants.  For  the  same  reasons,  groups  of  four  spores, 
(tetraspores),  are  developed  at  the  end  of  the  sporophyte  genera- 
tion in  the  red  algae. 


348  ALTERNATION  OF  GENERATIONS 

The  chromosomes  are  generally  believed  to  be  the  actual 
bearers  of  the  qualities  (represented  perhaps  by  rudiments) 
which  are  inherited,  that  is,  passed  on  from  one  generation  to  the 
next.  The  chief  reasons  for  this  view  are  their  importance  as 
the  essential  structures  of  the  nucleus,  their  regular  behavior 
throughout  the  cell  divisions,  and  the  evidence  that  they  never 
lose  their  identity  completely,  even  in  the  resting  nucleus,  but 
remain  perhaps  as  the  only  permanent  organs  in  the  cell. 

335.  The  life-history  formula,  showing  the  chromosome 
count.  The  life-history  formula  which  has  been  employed  for 
the  bryophytes  and  pteridophytes  becomes  much  more  interest- 
ing when  considered  in  reference  to  the  chromosome  count. 
The  formula  has  been  : 

Gametophyte  <^  ^>  —  Sporophyte  —  asexual  spore 

UtJ 

—  GametopJiyte,  etc. 

Representing  the  gametophyte  number  of  chromosomes  by  x 
and  the  sporophyte  number  by  2  a?,  these  may  accompany  the 
formula  as  follows : 

.  sperm 

Gametophyte  <^    x  chro"    ^>  —  Sporophyte  —  asexual  spore 
x  chromosomes      ^^     egg      ^         2 x  chromosomes       x.chromosomes 
x  chro. 

—  Gametophyte,  etc. 
x  chromosomes 

Examining  this  formula,  it  is  clear  that  there  are  two  periods 
when  the  number  of  chromosomes  changes  abruptly:  (1)  at  fer- 
tilization, when  the  number  is  doubled,  and  (2)  at  spore  forma- 
tion, when  the  number  is  reduced.  The  fertilized  egg  develops 
into  the  sporophyte  because  its  protoplasm  has  different  qualities 
from  that  of  the  gametophyte.  The  asexual  spore  develops  into 
the  gametophyte  because  its  qualities  have  become  again  the  same 
as  those  of  the  former  gametophyte  generation.  Spore  formation, 
then,  in  bryophytes  and  pteridophytes  is  a  return  of  the  plant 
in  its  life  history  to  the  conditions  of  ancestral  gametophytes. 


THE  ORIGIN  OF  THE  SPOROPHYTE  349 

336.  The  origin  of  the  sporophyte.    It  seems  clear  that  the 
sporophyte  had  its  origin  through  the  stimulus  of  the  union 
of  gametes,  and  especially  the  union  of  gamete  nuclei,  to  give 
a  fusion  nucleus  with  double  the  number  of  chromosomes  char- 
acteristic of  the  gametophytes.    It  is  probable  that  there  is  a 
reduction  of  this  number  in  many  thallophytes  before  or  during 
the  germination  of  the  zygospore  or  oospore,  so  that  there  is 
no  opportunity  for  a  sporophyte  generation.    This  condition  has 
been  reported  for  Coleochcete  (Sec.  222),  and  it  is  probably  also 
true  of  (Edogonium,  Spiroyyra,  the  desmids,  Vaucheria,  Ulothrix, 
and  other  types. 

The  sporophyte  arose  when  nuclear  divisions  appeared  with 
the  double  number  of  chromosomes,  thus  postponing  the  time 
of  chromosome  reduction  to  a  later  period  in  the  life  history, 
which  became  generally  characterized  by  the  formation  of 
asexual  spores  in  tetrads.  Sporophytes  undoubtedly  appeared 
thus  in  several  groups  of  plants  entirely  independently  of  one 
another,  as  illustrated  in  the  divergent  lines  of  development  of 
the  red  algae,  the  sac  fungi,  the  Dictyotacece  (a  small  group  of 
the  brown  algae),  and  the  bryophytes  leading  up  to  the 
pteridophytes  and  spermatophytes. 

337.  Summary.    The  alternation  of   generations    in  plants 
takes  on   added  interest  when   considered  in  relation  to  the 
behavior  of  the  chromosomes,  for  the  importance  of  the  two 
critical  stages  in  the  life  history —  (1)  fertilization,  and  (2)  spore 
formation —  becomes  at  once  apparent.    Fertilization  doubles  the 
number  of  chromosomes  in  the  egg  and  gives  it  the  possibilities 
of  the  sporophyte's  development.    Spore  formation  reduces  the 
double  number  of  chromosomes  by  half  and  brings  the  plant's 
protoplasm  back  to  the  condition  where  it  may  develop  the 
gametophyte.    The  two  processes  follow  one  another  as  the  life 
history  is  repeated  again  and  again  with  machine-like  regularity, 
and  there  is  undoubtedly  a  chemical  and  physical  basis  for  the 
life  history.    And,  as  before  stated,  it  is  generally  believed  that 
the  chromosomes  hold  the  rudiments  that  determine  in  a  broad 


350  ALTERNATION  OF  GENERATIONS 

way  the  programme  of  development,  the  double  number  defining 
the  sporophyte  generation. 

It  must  not  be  supposed,  however,  that  the  life  history  unfolds 
entirely  through  the  operation  of  forces  within  the  organism,  as 
a  watch  runs  on  the  strength  of  the  wound-up  mainspring. 
While  the  organism  is  truly  a  machine,  it  is  a  machine  which 
is  constantly  influenced  by  forces  from  without  which  modify 
its  complex  adjustments,  and,  above  all,  it  is  a  self-perpetuating 
machine  which  makes  its  own  repairs. 

There  are  two  prominent  theories  respecting  the  manner  in 
which  an  organism  develops  from  an  egg  or  other  reproductive 
cell.  The  first,  called  preformation,  assumes  that  the  characters 
of  the  adult  are  preformed  or  represented  in  miniature  by  rudi- 
ments or  other  structures  in  the  protoplasm.  Development  is, 
therefore,  something  like  the  unfolding  of  a  bud,  and  the  results 
are  determined  by  conditions  within  the  organism.  The  second 
theory,  termed  epigenesis,  is  not  willing  to  grant  such  a  compli- 
cated architecture  to  protoplasm,  but  holds  that  development 
is  guided  chiefly  by  conditions  without  the  organism.  It  is 
probable  that  the  correct  interpretation  lies  between  the  two 
extreme  views,  that  the  cell  does  have  a  complicated  structure 
far  beyond  our  present  possibilities  of  knowledge,  but  that  the 
processes  of  development  are  largely  guided  and  controlled  by 
outer  influences. 


CHAPTEE  XXVII 
HETEROSPORY 

338.  Heterospory.*  Heterospory  arose  in  the  pteridophytes 
with  the  establishment  of  two  sizes  of  spores,  called  megaspores 
(large  spores)  and  microspores  (small  spores).  Heterospory  and 
the  independence  of  the  sporophyte  are  the  chief  contributions 
of  the  pteridophytes  to  the  progress  of  plant  evolution.  The 
establishment  of  megaspores  and  microspores  was  merely  the 
beginning  of  a  number  of  far-reaching  developments  in  plants, 
all  of  which  are  really  parts  of  the  general  principle  of  hetero- 
spory.  They  all  reach  their  highest  degrees  of  specialization  in 
the  seed  plants,  as  will  be  described  in  the  next  chapter  and 
summarized  in  Chapter  xxix,  but  most  of  them  are  clearly 
illustrated  in  the  pteridophytes. 

These  developments  resulting  from  heterospory  are : 

1.  The  gametophytes   became   differentiated  in  sex  so  that 
the  megaspore  always  develops  a  female  gametophyte  and  the 
microspore  a  male  gametophyte. 

2.  The  sporangia  assumed  two   forms :    megasporangia  de- 
voted to  the   production  of    megaspores  and  microsporangia 
devoted  to  the  production  of  microspores,  as  illustrated  by  Mar- 
silia,  Isoetes,  Selaginella,  and,  as  will  appear  in  the  next  chapter, 
the  seed  plants. 

3.  The  spore  leaves,  or  sporophylls,  were  differentiated  into 
megasporopliylls  and  microsporopliylls  which   develop,   respec- 
tively, megasporangia   and    microsporangia,   as   illustrated   by 
Isoetes,  Selaginella,  and  the   seed  plants ;    the  sporophylls  of 
Marsilia  bear  both  forms  of  sporangia. 

*  To  THE  INSTRUCTOR  :  In  a  brief  course  or  with  immature  students  this 
chapter  should  be  omitted. 

351 


352  HETEROSPORY 

4.  A  tendency  was  developed  to  reduce  the  number  of  mega- 
spores  by  sacrificing  many  of  the  cells  which  might  be  fertile  so 
that  relatively  few  megaspores  are  formed,  but  these  are  very 
large  and  richly  supplied  with  food  material,  as  illustrated  by 
Selaginella  and  the  seed  plants.    This  principle  is  clearly  similar 
to  that  by  which  plants  have  found  it  advantageous  to  produce 
a  limited  number  of  large  eggs  well  stocked  with  food,  even  at 
the  sacrifice  of  cells  which  may  have  been  originally  gametes, 
such  as  the  canal  cells  in  the  archegonium. 

5.  The  gametophytes  degenerated,  as   self-supporting  green 
plants,  to  a  condition  in  which  they  lost  their  chlorophyll  and 
became  dependent  upon  food  stored  in  the  megaspores  and  mi- 
crospores  and  even  live  somewhat  parasitically  upon  the  sporo- 
phytes,  as  is  illustrated  in  the  early  stages  in  the  development  of 
the  female  gametophyte  of  Selaginella  and  in  the  gametophytes 
(pollen  tube  and  embryo  sac)  of  the  seed  plants. 

There  is  another  important  advance  in  plant  evolution  which 
is  closely  related  to  heterospory,  but  may  be  treated  to  better 
advantage  in  the  account  of  the  origin  of  the  seed  habit 
(Sec.  367).  This  advance  arose  in  the  seed  plants  when  the 
megaspore  became  retained  within  the  megasporangium  (a  por- 
tion of  the  ovule),  so  that  the  female  gametophyte  (embryo  sac) 
developed  like  a  parasite  upon  the  parent  sporophyte,  and  the 
male  gametophyte  (pollen  tube)  was  required  to  grow  down  to  the 
female  gametophyte  somewhat  parasitioally  through  the  tissues 
of  the  ovule  to  bring  about  the  fertilization  of  the  egg  cell. 

339.  Sexual  characteristics  given  to  the  megaspore  and  mi- 
crospore  by  means  of  heterospory.  The  megaspore  and  micro- 
spore  are  of  course  asexual  spores  because  they  are  formed  by 
an  asexual  plant,  the  sporophyte.  They  are  simply  specialized 
forms  of  the  similar  spores  present  in  the  liverworts,  mosses, 
the  common  ferns,  horsetails,  and  lycopods,  as  shown  by  their 
similar  origin  in  tetrads  at  the  end  of  the  sporophyte  generation. 

But  when  the  microspore  and  megaspore  became  clearly  dif- 
ferentiated through  heterospory  from  the  earlier  conditions  of 


SEXUAL  CHARACTERISTICS  GIVEN  BY  HETEROSPORY    353 

homospory  they  took  on  certain  characteristics  of  sex.  This  does 
not  mean  that  the  microspores  and  megaspores  became  gametes, 
for  their  spore  nuclei  have  never  become  sexual  nuclei  in  any 
group  of  plants.  But  microspore  and  megaspore  did  assume 
sexual  characters  to  this  extent  that  they  always  give  rise, 
respectively,  to  male  and  female  gametophytes. 

Furthermore,  the  degeneration  of  the  gametophytes  steadily 
reduced  the  number  of  the  nuclear  divisions  between  the  germ- 
ination of  these  spores  and  the  formation  of  the  gametes  until 
the  gamete  nuclei  have  been  brought  very  close  indeed  to  the 
spore  nuclei.  An  examination  of  the  figures  of  the  male  game- 
tophytes of  Marsilia  (Fig.  282,  A},  Selaginella  (Fig.  290,  A),  and 
Isoetes  (Fig.  292,  A)  will  show  that  there  can  hardly  be  more 
than  from  six  to  ten  nuclear  divisions  in  these  types  before  the 
sperms  are  developed.  There  are  even  fewer  nuclear  divisions 
in  some  groups  of  seed  plants  where  the  degeneration  of  the 
gametophyte  is  carried  much  further  than  in  the  pteridophytes. 
Some  forms  of  angiosperms  present  but  a  single  division  of  the 
spore  nucleus  before  the  female  gamete  nuclei  are  formed,  as 
in  the  embryo  sac  of  the  lily  (Sec.  360,  note),  and  there  are  only 
two  nuclear  divisions  in  the  male  gametophytes  (pollen  grain 
and  tube)  of  the  angiosperms. 

This  gradual  transference  of  sexual  characteristics  to  portions 
of  the  asexual  generation,  accompanying  the  reduction  of  the 
sexual  generation,  is  one  of  the  most  interesting  results  of  the 
evolution  of  the  sporophyte  and  degeneration  of  the  gameto- 
phyte (summarized  in  Chapter  xxix),  for  it  makes  clear  many 
puzzling  conditions  in  the  seed  plants.  Thus  it  shows  why  the 
pollen  grain  (which  is  a  microspore)  is  really  functionally  a  male 
reproductive  structure  and  the  stamen  a  male  organ ;  and  why 
the  carpels  and  pistil  are  functionally  female  organs,  even  though 
they  have  had  their  origin  on  asexual  plants  (sporophytes). 


CHAPTER  XXVIII 
THE  SPERMATOPHYTES  AND  THE  SEED  HABIT 

340.  The  spermatophytes.*  The  division  Spermatophyta 
(meaning  seed  plants)  contains  not  only  the  groups  frequently 
called  "  flowering  plants  "  but  also  other  groups  which  do  not 
have  flowers  in  the  popular  sense  of  the  word,  for  the  repro- 
ductive organs  are  .borne  in  cones  or  clusters  which  are  not 
at  all  showy,  but  rather  inconspicuous.  These  are,  however, 
flowers  in  the  scientific  sense,  as  are  also  the  cones  of  the 
horsetails  and  club  mosses.  The  spermatophytes  have  also  been 
called  phanerogams,  or  plicenogams  (meaning  evident  marriage), 
to  distinguish  them  from  all  the  lower  groups  of  plants  which 
were  called  cryptogams  (meaning  hidden  marriage).  However, 
this  separation  was  made  before  the  sexual  processes  of  the 
lower  plants  were  understood,  for  as  a  matter  of  fact  they  are 
much  more  evident  than  the  complicated  ones  in  the  seed  plants. 
The  seed  is  a  more  significant  structure  in  the  group  than  the 
flower,  so  the  name  spermatophytes  has  in  recent  years  come 
into  general  favor. 

The  seed  plant,  like  the  fern,  is  a  sporophyte.  There  is  a 
gametophyte  generation  in  the  life  history  which  is,  however,  so 
much  reduced  in  structure  that  it  can  only  be  understood 
by  a  careful  study  of  the  reproductive  processes  in  seed  for- 
mation. It  is  the  main  purpose  of  this  chapter  to  make  clear 
the 'position  of  the  gametophyte  generation  in  the  life  history, 
together  with  the  origin  and  evolution  of  the  flower.  The  struc- 
ture and  physiology  of  the  sporophyte  are  considered  in  Part  I, 

*  To  THE  INSTRUCTOR  :  The  introduction  to  this  chapter  assumes  that  the 
life  history  of  some  seed  plant,  as  the  pine  or  lily,  has  been  studied  in  the 
laboratory. 

354 


THE  SEED  355 

and  only  brief  reference  will  be  made  to  these  features,  which 
are  treated  there  in  detail  and  should  follow  this  account  if  they 
have  not  already  been  studied. 

341.  The  seed.    The  importance  of  the  seed  in  the  develop- 
ment of  plant  and  also  of  animal  life  can  hardly  be  exaggerated. 
For  the  plant  it  furnishes  one  of  the  surest  means  of  reproduc- 
tion not  only  because  of  protective  structures,  means  of  dispersal, 
long  vitality,  etc.   (see   Chapter  xxxin),  but  also  because  the 
embryo  plant  is  carried  so  far  forward  in  its  development  that 
it  is  able  to  take  root  and  establish  itself  at  once.    And  fur- 
ther to  aid  the  embryo,  the  seed  is  a  storage  organ  of  the  most 
condensed   forms  of   food  material  found  in   plants.     In   this 
respect,  also,  the  seed  has  proved  a  most  important  influence  in 
shaping  the  habits  and  in  a  large  measure  the  evolution  of  some 
forms  of  animal  life ;  for  the  highest  groups  of  animals  live  to 
a  very  great  extent  directly  or  indirectly  upon  food  stored  in 
seeds  and  certain  fruits,  finding  there  some  of  the  richest  and 
most  nutritious  proteid  and   carbohydrate  foods.    The  animal 
life  of  the  Carboniferous  Age  (coal  age)  and  the  periods  imme- 
diately following  comprised  animals  of  great  bulk  of  body,  but  of 
low  nervous  organization.    They  browsed  on  the  vegetation  like 
the  hay  and  grass-eating  animals  (herbivora)  of  to-day,  and  like 
them  their  bodily  structure  and  nervous  system  were  adapted 
to  such  life  habits.    But,  later,  groups  arose  with  digestive  organs 
adapted  to  richer  foods,  and  this  diet  became  associated  with 
varied  life  habits,  which  led  ,to  much  higher  types  of  nervous 
organization  and  bodily  structure. 

342.  The  morphology  of  the  seed.    The  morphology  of  the 
seed   can  only   be  understood  when    the  spermatophytes  are 
studied  in  relation  to  the  pteridophytes.    The  seed  plant  is  a 
heterosporous  sporophyte.    The  pollen  grain  is  a  microspore. 
The  megaspores  of  the  seed  plant  are  never  shed.    They  are 
retained  in  the  megasporangium  and  never  even  lie  freely  as 
independent  cells,  but  are  always  in  close  physiological  relation 
to  the  tissue  of  the  megasporangium.    The  cell  which  is  the 


356  THE  SPERMATOPHYTES 

equivalent  of  the  megaspore,  or  rnegaspore  mother  cell,  is  called 
the  embryo  sac.  The  megasporangium,  termed  the  nucellus, 
with  the  embryo  sac  is  contained  within  one  or  two  protective 
envelopes,  called  integuments,  and  this  group  of  structures  con- 
stitutes the  ovule.  There  is  developed  within  the  embryo  sac  a 
much-reduced  female  gametophyte  which  lives  entirely  on  foods 
supplied  by  the  sporophyte.  The  ovule  at  maturity  then  con- 
sists of  the  embryo  sac  (megaspore  or  rnegaspore  mother  cell) 
with  the  female  gametophyte,  the  nucellus  (megasporangium), 
and  the  integuments. 

The  female  gametophytes  are  quite  different  in  the  two  great 
subdivisions  of  seed  plants  (gymnosperms  and  angiosperms). 
In  the  first  group  (gymnosperms)  several  archegonia  are  gener- 
ally, formed,  each  containing  a  single  large  egg.  In  the  second 
group  (angiosperms)  the  female  gametophyte  is  very  much  re- 
duced and  only  one  egg  is  formed.  The  fertilization  of  an  egg 
leads  at  once  to  the  development  of  an  embryo  sporophyte 
within  the  embryo  sac.  The  embryo  sporophyte  of  the  second 
generation  is  thus  nourished  through  the  ovule  by  the  parent 
sporophyte  of  the  first. 

The  seed  is  a  ripened  ovule,  that  is,  an  ovule  containing  an 
embryo  sporophyte  so  far  along  in  its  development  that  the  seed 
may  safely  be  separated  from  the  parent  plant.  Morphologically, 
the  seed  is  composed  of  tissues  representing  three  generations: 

(1)  the  integuments  and  nucellus  are  of  the  parent  sporophyte; 

(2)  the  embryo  sac  contains  more  or  less  tissue  of  gametophyte 
origin  called  endosperm  l;  (3)  an  embryo  sporophyte  of  the  next 
generation  lies  within  the  embryo  sac. 

343.  Pollination  and  fertilization.  The  retention  of  the  mega- 
spore (embryo  sac)  within  the  megasporangium  (nucellus)  so 
that  the  female  gametophyte  is  contained  in  the  tissues  of  the 
sporophyte  has  resulted  in  modifications  of  the  structure  and 

1  The  endosperm  of  the  angiosperm  seed  has  special  peculiarities  involved 
•with  the  fertilization  of  the  egg  and  development  of  the  embryo,  as  explained 
in  Sees.  362  and  363. 


POLLINATION  AND  FERTILIZATION  357 

life  habits  of  the  male  gametophyte  quite  as  remarkable  as  those 
of  the  female.  These  peculiarities  are  concerned  with  two  dis- 
tinct processes  necessary  to  insure  the  development  of  seeds, 
namely,  pollination  and  fertilization. 

The  pollen  grain  is  a  microspore  developed  in  groups  of  four 
(tetrads)  in  pollen  mother  cells  in  essentially  the  same  manner 
as  the  spores  are  developed  in  all  bryophytes  and  pteridophytes 
(Fig.  302,  B).  The  pollen  grain  forms  a  very  much  reduced 
male  gametophyte,  which  is  represented  by  the  protoplasmic 
contents  of  the  pollen  grain  and  pollen  tube.  It  would  be  use- 
less Tor  the  male  gametophyte  to  form  and  discharge  sperms 
which  could  not  possibly  reach  the  embryo  sac  imbedded  in  the 
nucellus  of  the  ovule.  So  the  sperm-forming  habits  of  the 
pteridophytes,  bryophytes,  and  the  algse  are  generally  given  up, 
although  curiously  they  still  persist,  as  will  be  described  later, 
in  the  cycads  and  Ginkgo  (Sec.  348).  The  sperms  are  repre- 
sented by  two  sperm  nuclei  developed  by  each  male  gameto- 
phyte and  discharged  from  the  tip  of  the  pollen  tube. 

The  pollen  tube  is  an  outgrowth  from  the  pollen  grain.  Its 
purpose  is  to  carry  the  sperm  nuclei  to  the  embryo  sac,  where 
one  of  the  two  may  unite  with  the  egg  nucleus  and  fertilize  the 
egg.  In  one  of  the  two  subdivisions  of  seed  plants  called  the 
gymnosperms  (meaning  naked  seeds)  the  pollen  grains  are 
applied  directly  to  the  ovules,  and  the  pollen  tube  need  only 
grow  through  the  tissue  of  the  nucellus  (megasporangium)  to 
reach  the  embryo  sac.  In  the  other  large  group  called  the  angio- 
sperms  (meaning  seeds  inclosed  in  a  vessel)  the  pollen  tubes 
must  penetrate  a  case  (the  pistil)  which  contains  the  ovules 
before  they  can  reach  the  ovules  themselves.  There  is  a  special 
receptive  surface,  called  the  stigma,  upon  this  structure,  where 
the  pollen  grains  find  moisture  and  other  conditions  favorable 
for  their  germination. 

Pollination  is  the  application  of  the  pollen  to  the  ovule  or  to  the 
stigma.  This  application  is  effected  in  various  ways,  sometimes 
by  the  wind,  sometimes  by  other  chance  processes,  but  many 


358  THE  SPERMATOPHYTES 

plants  have  developed  elaborate  devices  to  insure  pollination,  as 
through  the  visits  of  insects  to  flowers. (see  Chapter  xxxn). 

Fertilization  is  effected  when  the  pollen  tube  pierces  the  embryo 
sac  and  one  of  its  two  sperm  nuclei  fuses  with  the  egg  nucleus. 
When  one  considers  the  extraordinary  modifications  of  the 
male  gametophytes  of  the  seed  plants,  the  process  of  pollination 
and  the  development  of  the  pollen  tube  seem  quite  as  remark- 
able as  the  retention  of  the  female  gametophyte  in  the  mega- 
sporangium.  They  are  both  essential  features  of  the  seed  habit. 
344.  The  flower.  The  term  flower  in  the  popular  sense  gen- 
erally means  some  showy  structure  such  as  is  only  found  in 
certain  groups  of  the  angiosperms.  The  flower  in  the  scientific 
sense  consists  of  a  group  of  spore  leaves,  or  sporophylls,  with 
or  without  surrounding  envelopes,  which  may  or  may  not  be 
showy.  It  has  been  defined  as  "  a  shoot  beset  with  sporophylls." 
Since  the  seed  plants  are  heterosporous,  the  spore  leaves  are 
either  microsporophylls,  called  stamens  (producing  pollen] ,  or 
they  are  megasporophylls,  called  carpels  (producing  embryo  sacs 
in  the  ovules).  The  stamens  and  carpels  of  the  gymnosperms 
are  generally  grouped  in  cones  which  resemble  the  cones  of  the 
horsetails  and  club  mosses.  But  the  carpels  of  the  angiosperms 
form,  often  with  adjacent  tissue,  closed  cases  called  pistils. 

It  should  be  noted  that  the  cones  of  the  horsetails  and  club 
mosses  are  as  truly  flowers  in  the  scientific  sense  as  the  cones 
of  the  gymnosperms,  and  also  that  certain  groups  of  angio- 
sperms (grasses,  sedges,  and  most  trees)  have  flowers  which  are 
not  showy. 

The  material  of  this  chapter  will  be  treated  under  the  follow- 
ing headings : 

Subdivision  I.    The  gymnosperms,  or  Gymnospermce. 

Subdivision  II.  The  angiosperms,  or  Anyiospermce. 

The  origin  of  seed  plants  and  the  seed  habit. 

The  evolution  of  the  flower. 

The  classification  of  the  angiosperms. 

Summary  of  the  spermatophytes  and  their  relationships  to  the 
pteridophytes. 


THE  GYMNOSPERMS  359 

SUBDIVISION  I.    THE  GYMNOSPEKMS,  OR 
G  YMNOSPERM^E 

345.  The  gymnosperms.  The  gymnosperms  (meaning  naked 
seeds)  are  distinguished  from  the  angiosperrns  because  their 
seeds -are  borne  exposed  on  the  carpels.  They  comprise  not 
only  the  familiar  cone-bearing  trees,  or  conifers,  generally  with 
needle-shaped  leaves,  such  as  the  pines,  spruces,  firs,  hemlocks, 
larches,  cedars,  etc.,  but  also  the  large-leaved  cycads,  the 
straggling,  shrubby  Ephedras,  the  climbing  Gfnetums,  and  that 
interesting  Japanese  tree  Ginkgo.  The  gymnosperms  contain 
the  most  ancient  groups  of  living  seed  plants,  and  the  fossil 
remains  of  primitive  types  are  found  in  the  Carboniferous  Age 
and  even  earlier  periods,  with  those  of  the  giant  horsetails  and 
club  mosses  (see  Cordaites,  Plate  VIII).  Tlie  study  of  ancient 
gymnosperms,  together  with  a  fossil  group,  Ptcridospermoe, 
intermediate  between  the  pteridophytes  and  spermatophytes, 
may  throw  much  light  on  the  origin  of  the  seed  and  seed  habit. 
.  The  living  groups  of  the  gymnosperms  comprise  in  all  less 
than  450  species,  of  which  more  than  300  are  conifers  and 
about  80  are  cycads.  With  the  exception  of  the  conifers,  these 
groups  are  hardly  more  than  remnants  of  the  ancient  gymno- 
sperm  floras.  But  the  conifers  are  a  very  successful  group, 
which  still  forms  extensive  forests  in  some  temperate  regions 
and  covers  mountain  sides  and  certain  large  rather  barren  areas, 
although  such  forests  are  being  rapidly  cut  off  for  timber.  Of 
the  smaller  groups  the  cycads  are  mostly  tropical,  the  Ephedras 
are  chiefly  desert  plants,  and  the  Gnetums  tropical  vines  with 
large-veined  leaves.  Like  the  horsetails  and  club  mosses,  the 
Epliedras  have  for  the  most  part  developed  peculiar  life  habits 
under  unfavorable  conditions,  and  so  have  been  able  to  avoid 
total  extinction .  by  withdrawing  as  far  as  possible  from  compe- 
tition with  the  more  recent  floras. 

This  account  can  only  consider  the  two  largest  groups,  the 
cycads  and  the  conifers. 


360 


THE  SPERMATOPHYTES 


THE  CYCADS 

346.  The  cycads.  The  cycads  (order  Cycadales)  have  thick 
stems  which  rarely  branch  and  are  generally  rather  short,  resem- 
bling immense  tubers  partly  buried  in  the  ground  (Fig.  293,  A). 


A,  plant  bearing  a 
carpellate  cone.  B, 
a  carpel  in  side 
view,  showing  the 
two  very  large 
ovules:  ra,  mycro- 
pyle.  —  A,  adapted 
from  a  photograph 
by  Land 


FIG.  293.  A  cycad  (Zamia) 

Some  of  the  cycads  have,  however,  stems  which  rise  like  col- 
umns ten  to  forty  feet  high.  The  compound  leaves,  like  immense 
stiff  feathers,  form  a  crown  at  the  top  of  the  stem  so  that  the 
general  habit  of  the  cycads  is  somewhat  like  that  of  the  tree 


THE  CONES  OF  THE  CYCADS 


361 


ferns  and  palms.  One  form  (Cycas  revoluta),  incorrectly  called 
the  sago  palm  (since  it  is  not  a  palm),  is  valuable  for  the  sago 
of  commerce  which  is  obtained  from  the  stem. 

347.  The  cones  of  the  cycads.  Some  of  the  cycads  bear 
cones  composed  either  of  carpels  (megasporophylls),  or  stamens 
(microsporophylls)  which  resemble  large  scales.  Carpellate  and 
staminate  cones  are  always  borne  on 
separate  plants.  In  other  types,  how- 
ever, as  Cycas  rcvoluta,  the  carpels, 
especially,  have  more  nearly  the  ap- 
pearance of  vegetative  leaves  (Fig. 
294),  and  form  rosettes  at  the  top  of 
the  stems.  Cycas  revoluta  is  fre- 
quently grown  in  park  conservatories, 
and  occasionally  produces  these  ro- 
settes of  hairy,  orange-colored  carpels, 
which  bear  a  series  of  ovules  as  large 
as  plums  on  either  side.  Well-differ- 
entiated cones  are  present  in  Zamia 
(Fig.  293,  A),  which  is  quite' common 
in  southern  Florida.*  The  carpel 
(Fig.  293,  J5),  in  this  genus,  bears  two 
ovules  and  the  stamen,  a  group  of 
pollen  sacs  (Fig.  295,  A).  The  ovule 
(Fig.  295,  D)  has  a  thick  integument  FlG-  294.  Carpel  of  Cycas  revo- 
surrounding  the  large  nucellus,  in 

which  lies  the  embryo  sac  containing  the  female  gametophyte. 
The  pollen  grains  of  Zamia  enter  the  opening  called  the  micro- 
pyle  (meaning  little  gate),  where  the  integument  fails  entirely 
to  inclose  the  nucellus,  and  so  come  to  lie  in  a  small  cavity 

*  To  THE  INSTRUCTOR  :  It  ought  to  be  possible  to  obtain  Zamia  in  quantities 
for  advanced  classes.  The  type  is  most  admirable  for  the  study  of  the 
gametophytes  of  gymhosperms.  The  best  account  of  these  is  given  by  Web- 
ber, United  States  Department  of  Agriculture,  Bureau  of  Plant  Industry, 
Bulletin  2,  1901. 


362 


THE   SPERMATOPHYTES 


termed  the  pollen  chamber  (Fig.  295,  D,p).  The  pollen  grains 
germinate  in  the  pollen  chamber,  forming  male  gametophytes. 
whose  development  disorganizes  much  of  the  tissue  at  the  tip 

of  the  nucellus,  so  that  the 
pollen  grain  end  of  the  male 
gametophytes  finally  hang 
down  just  above  the  em- 
bryo sac. 

348.  The  gametophytes 
of  the  cycads.  The  embryo 
sac  of  the  cycads  is  said  to 
develop  from  one  of  a  group 
of  four  cells  in  the  interior 
of  the  nucellus.  Such  a 
group  is  undoubtedly  a 
tetrad,  and  each  of  the  four 
cells  corresponds  to  a  mega- 
spore,  but  only  one  produces 
a  female  gametophyte,  and 
thus  becomes  an  embryo  sac. 
The  nucleus  of  the  em- 
bryo sac  (megaspore  nucleus) 
gives  rise  to  a  great  many 


FIG.  295.  The  sperms  and  ovule  of  a 
cycad  (Zamia) 

A,  lower  surface  of  a  stamen,  with  numer- 
ous pollen  sacs  in  two  groups.    JJ,  the  two 


hundred   nuclei,  and   the 


large  top-shaped  motile  sperms  at  the  end     amount     of     protoplasm     ill- 


creases   very    greatly    until 
the  embryo  sac  occupies  the 


of  the  pollen  tube  ready  to  be  discharged 

above  the  archegonia.    C,  a  sperm  viewed 

from  the  end,  showing  the  spiral  band 

which  bears  the  cilia.    D,  diagram  of  a     -  P     , 

section  of  an  ovule  after  pollination :  m,     larger    Parl 

micropyle;  i, integument;  p, pollen cham-     of  the  nucellus  in  this  large 

her;    n,  nucellus  containing  developing  ,        ml  n    .      ,    n      ,  -,. 

pollen  tubes;  a,  archegonia,  with  large     ovule.    The  nuclei  at  first  lie 

eggs  imbedded  in  the  endosperm  (female     freely  in  the  protoplasm,  but, 

gametophyte).  —  B,  C,  after  Webber  ,,  ,  ,          , 

later,  walls  are  formed  and 

the  embryo  sac  becomes  filled  with  a  delicate  tissue,  called  the 
endosperm  (Fig.  295,  D),  which  corresponds  to  the  vegetative  part 
of  a  prothallium  in  a  fern.  Several  archegonia  are  developed  at 


THE  GAMETOPHYTES   OF   THE  CYCADS  363 

the  micropylar  end  of  the  endosperm  (Fig.  295,  D,  a).  These  are 
very  much  reduced  in  structure,  the  neck  being  represented  prob- 
ably by  two  small  cells  and  the  very  large  eggs  lying  imbedded 
in  the  cells  of  the  endosperm. 

The  male  gametophyte  consists  of  the  protoplasm  with 
several  nuclei  contained  in  the  pollen  grain  and  tube.  Some  of 
the  nuclei  near  the  pollen  grain  end  of  the  tube  lie  within  deli- 
cate cell  walls.  One  of  these  cells  termed  the  generative  cell 
develops  two  sperm  mother  cells  which  become  organized  into 
two  very  large  motile  sperms  (Fig.  295,  B,  C),  each  with  a  spiral 
band  or  line  bearing  hundreds  of  cilia.  The  two  sperms  finally 
begin  to  move  around  in  the  fluid  of  the  pollen  tube  and  are 
discharged  from  the  end  nearest  the  pollen  grain  (which  now 
hangs  down  over  the  embryo  sac)  into  the  fluid  within  the 
cavity  formed  from  the  disorganized  tissue  at  the  tip  of  the 
nucellus.  The  pollen  tube  in  the  cycads  grows  off  to  one  side 
in  the  nucellus  and  seems  to  be  a  sort  of  absorbing  organ,  so 
that  it  does  not  carry  the  sperms  to  the  embryo  sac  as  the 
sperm  nuclei  are  carried  in  most  seed  plants. 

The  motile  sperms  are  set  free  in  the  fluid  above  the  embryo 
sac,  whose  female  gametophyte  at  that  time  bears  mature  arche- 
gonia.  They  have  been  observed  swimming  about  for  many  min- 
utes in  sections  of  the  living  ovules,  and  probably  have  a  long 
motile  period  in  the  ovule.  One  of  them  is  finally  able  to  enter 
the  neck  of  an  archegonium,  and  fusing  with  an  egg  fertilizes  it. 

The  finding  of  motile  sperms  in  the  cycads  and  in  Ginkgo1 
fyy  two  Japanese  botanists  in  1896-1897  proved  two  of  the  most 
interesting  botanical  discoveries  of  the  past  decade.  It  is  very 
remarkable  that  the  sperm-forming  habits  of  the  bryophytes  and 
pteridophytes  should  have  persisted  so  long  after  the  seed  habit 
became  established  in  a  group.  The  free  swimming  of  these 
motile  sperms  is  actually  a  return,  such  as  occurs  in  the  bryo- 
phytes and  pteridophytes,  for  a  short  time  in  the  life  history  of  the 
cycad  to  the  aquatic  habits  of  an  algal  ancestry  of  ages  ago. 

1  A  beautiful  Japanese  tree,  not  uncommon  under  cultivation. 


364  THE   SPERMATOPHYTES 

THE  CONIFERS 

349.  The  conifers.    This  group  (order  Coniferales)  has  repre- 
sentatives distributed  all  over  the  earth,  some  of  them  forming 
the  most  extensive  forests  and  having  the  greatest  value  as 
timber  trees.    There  are  not  many  more  than  300  species  of 
conifers,   of   which    the   pines    (Pinus)  have   70;    Podocarpus 
(growing  in  South  America  and  eastern  Asia),  40 ;  the  junipers 
(Jnniperus),  30;  certain  cedars  (Cnpressw),  20;  the  firs  (Abies), 
20;  and  the  spruces  (Picea),  12.    Others  have  few  species  and 
a  very  limited  distribution.    Such  a  form  is  the  giant  redwood 
of  California  (Sequoia  gig  anted),  which  is  found  only  in  a  few 
scattered  groves  in  the  Sierra  Nevada  Mountains  (Fig.  33). 

350.  The  form  and  foliage  of  the  conifers.    The  form  and 
foliage  of  the  conifers  is  •  generally  very  characteristic.    The 
trees  have,  as  a  rule,  a  single  central  stem  which  rises  vertically 
from  the  ground,  and  the  side  branches  spread  out  horizontally 
from  this  shaft  so  that  the  trees  are  very  symmetrical  and  taper 
to  a  point  like  a  cone.    The  foliage,  as  a  rule,  consists  of  scale- 
or  needle-shaped  leaves,  which  usually  remain  on  the  trees  for 
a  number  of  years  so  that  most  of  the  trees  are  evergreen.    But 
there  are  some  exceptions  to  the  rule,  as  the  larch  or  tamarack 
(Larix),  which  sheds  its  needles  every  year. 

The  needle  leaves  can  endure  severe  cold,  fierce  heat,  and 
drought.  This  is  made  possible  by  their  very  compact  structure 
(Fig.  296),  which  presents  a  minimum  of  surface  exposure  and 
the  protective  layer  of  thick-walled  cells  under  the  heavy  epideii- 
mis.  The  chlorophyll-bearing  tissue  is  closely  packed  in  the  pine 
leaf  and  consists  of  cells  with  peculiar  infolding  walls.  Some 
species  of  pine  have  needles  with  one  fibro-vascular  bundle,  e.g. 
the  white  pine;  others  have  two  bundles,  e.g.  the  Scotch  and 
the  Austrian  pine.  The  buds,  leaves,  and  stems  contain  much 
resin  and  turpentine,  which  render  them  unpalatable  to  grazing 
animals  and  cover  them  with  a  film  which  sheds  water  and 
protects  the  plant  both  from  the  winter's  cold  and  the  summer's 


THE  FORM  AND  FOLIAGE   OF   THE  CONIFERS      365 

drought.  Resins  and  turpentines  are  also  very  effective  in  pro- 
tecting young  conifers  from  the  attacks  of  parasitic  fungi,  espe- 
cially when  the  trees  are  wounded. 

Certain  pines  furnish  the  resin  and  turpentine  of  commerce. 
Incisions  are  made  through  the  bark,  penetrating  the  wood. 
A  thick  liquid  oozes  out  which  is  a  mixture  of  resins  and  oil  of 
turpentine.  This  liquid  is  then  distilled,  driving  off  the  fluid 
oil  of  turpentine  which  is  collected.  The  resin  remains  behind 


FIG.  296.  Structure  of  a  pine  needle  (Pinus  Laricio) 

The  compact  green  tissue,  or  mesophyll,  with  resin  ducts  d,  surrounds  an  area  con- 
taining two  fibro-vascular  bundles,  which  lie  in  a  peculiar  region  of  transfu- 
sion tissue  t,  bounded  by  the  bundle  sheath  66'.  Outside  of  the  green  tissue  are 
thick-walled  cells  forming  a  rigid  tissue  r,  and  around  the  whole  is  the  heavy 
epidermis  e  with  lengthwise  grooves  containing  the  stomata  st 

in  the  still,  and  when  cool  is  no  longer  semi-fluid,  but  becomes 
quite  hard  and  brittle.  The  timber  value  of  certain  conifers  is 
much  greater  than  that  of  most  other  kinds  of  trees  because  the 
wrood  is  soft,  splits  regularly,  is  easily  worked,  and  also  because 
the  tree  trunks  are  so  straight.  The  problems  of  forestry  (see 
Chapter  XLI)  are  largely  concerned  with  the  preservation  of  the 
pine  forests,  which  are  being  cut  off  with  little  regard  to  the  future. 


366 


THE   SPERMATOPHYTES 


351.  The  tissues  of  the  pine  stem.  The  pine  is  an  excellent 
subject  for  the  study  of  stem  structure  and  growth  in  a  timber 
tree.  There  are  five  principal  regions  in  the  stem:  (1)  the  pith, 


FIG.  297.   Structure  of  the  stein  of  the  Scotch  pine  (Pinus  sylvestris) 

A,  diagram  of  the  arrangement  of  the  fibre-vascular  bundles  at  a  growing  point : 
the  shaded  parts  are  wood.  .Bt  diagram  of  the  position  of  the  principal  tissues 
shown  in  a  cross  section  of  a  four-year-old  stem.  C,  cross  section  of  a  region 
of  cambium  Cam,  with  adjacent  wood  and  bast.  D,  cross  section  of  wood  at 
an  annual  ring:  d,  resin  duct.  E,  radial  section  of  wood.  F,  longitudinal  sec- 
tion of  wood.  G,  section  of  bordered  pit.  Medullary  rays  m  appear  in  most 
of  tbe  figures 

(2)  the  wood,  (3)  the  cambium,  (4)  the  bast,  and  (5)  the  outer 
bark  (see  Fig.  297,  #). 

The  pith  occupies  the  very  center  of  the  stem,  and  is  the 
remains  of  the  undifferentiated  primitive  tissue  present  at  its 


THE  TISSUES  OF  THE  PINE  STEM  367 

growing  point  before  the  fibro-vascular  bundles  and  bark  are 
formed.  It  practically  disappears  as  the  stem  grows  older  and 
the  wood  increases  by  a  number  of  years  of  annual  growth. 

The  wood,  or  xylem,  comprises  by  far  the  greater  part  of  older 
stems,  becoming  proportionally  greater  as  each  annual  ring  is 
added.  It  is  composed  of  very  much  elongated'  cells,  called 
tracheids,  with  firm,  somewhat  yellowish,  thick  walls.  Cell 
walls-  of  this  character  are  said  to  be  lignified.  These  cells  con- 
tain pits  (Fig.  297,  E,  F,  G)  surrounded  by  a  circle  and  termed 
bordered  pits,  the  circle  being  a  feature  characteristic  of  this 
group  of  plants.  There  are  resin  ducts  among  the  wood  cells, 
and  also  peculiar  plates  of  cells  called  medullary  rays  which 
extend  through  the  cambium  and  bast  into  the  outer  wood.  The 
medullary  rays  have  the  form  of  thin  knife  blades  penetrating 
the  wood  for  various  distances. 

The  cainbium  is  a  cylinder  of  thin- walled  dells  just  outside 
of  the  wood,  and  is  the  most  active  region  of  growth  in  the 
stem.  This  cylinder  (Fig.  297,  C)  is  only  two  or  three  cells 
wide,  and  the  cells  are  continually  dividing  by  walls  parallel  to 
the  surface  (tangentially)  during  the  season  of  growth.  The 
-daughter  cells  on  the  inside  of  the  cambium  become  firm  wood 
cells  by  the  thickening  of  their  walls  together  with  certain 
changes  (lignification)  that  give  them  firmness  ;  they  also  become 
empty  of  protoplasm.  The  daughter  cells  on  the  outside  of  the 
cambium  form  the  bast,  remaining  soft  and  containing  proto- 
plasm and  much  food  material.  The  cambium  thus  adds  cells  to 
the  wood  on  the  inside  and  the  bast  on  the  outside.  The  wood  is 
deposited  in  annual  rings  during  the  season  of  growth,  and  these 
are  sharply  distinguished  from  one  another  because  the  wood 
cells  formed  at  the  beginning  of  one  season  are  larger  than  those 
formed  in  the  latter  part  of  the  previous  season  (Fig.  297,  D). 

The  last  is  difficult  to  study  chiefly  because  the  cells  are 
under  severe  pressure  from  the  growing  cambium  on  the  inside 
and  the  restraining  bark  on  the  outside,  and  the  cell  arrange- 
ments are  frequently  distorted. 


368 


THE   SPERMATOPHYTES 


The  outer  lark  is  developed  from  the  primitive  or  ground 
tissue  which  lay  outside  of  the  circles  of  wood  and  bast  when 
these  circles  were  first  formed  by  the  union  of  the  primary 
fibro-vascular  bundles  (Fig.  297,  A),  as  described  in  Sec.  79. 
There  is  much  actively  growing  tissue  in  the  bark,  but  the  outer 


FIG.  298.   The  staminate  cone,  stamen,  and  pollen  of  the  Scotch  pine 
(Pinw.s  sylvestris) 

A,  young  growth,  with  staminate  cones  about  two  weeks  after  the  opening  of  the 
terminal  hud.  B,  details  of  cone.  C,  end  view  of  stamen.  D,  side  view  of 
stamen.  E,  pollen  mother  cell  developing  four  pollen  grains  in  a  tetrad.  F, 
pollen  grain  showing  the  two  wings :  p,  prothailial  'tell ;  g,  generative  cell  ; 
t,  tube  nucleus.  —  E,  after  Miss  Ferguson 

regions  become  quite  dead,  and  crack  under  the  pressure  of  the 
growing  cambium,  thus  forming  scales.  The  cracks  are  healed 
by  the  living  tissue  of  the  bark.  The  bast  is  generally  so  closely 
attached  to  the  outer  bark  that  it  peels  off  with  it,  and  therefore 


THE   STAMINATE   CONE 

is  a  sort-  of  inner  bark  and  must  be  included  in  any  account 
of  this  region  of  the  stem. 

The  functions  of  these  tissues  are  discussed  in  Part  I, 
Chapter  vm. 

352.  The  cones  of  the  pine.  The  cones  of  the  pine,  as  in  all 
conifers,  are  of  two  sorts:  (1)  staminate,  when  made  up  of  stamens 
(microsporophylls),  and  (2)  carpellate,  when  composed  of  carpels 
(rnegasporophylls). 

The  staminate  cone.  The  staminate  cones  are  developed  in 
clusters  on  the  young  growth  that  appears  late  in  the  spring 
with  the  opening  of  the  terminal  buds  (Fig.  298,  A).  Each 
cone  consists  of  a  large  number  of  stamens  closely  packed 
together  and  arranged  somewhat  spirally  around  the  central  axis 
(Fig.  298,  B).  The  stamen  bears  two  pollen  sacs  (Fig.  298,  C,  D), 
within  which  the  pollen  grains  are  developed.  The  pollen  grains 
are  formed  in  groups  of  four,  or  tetrads  (Fig.  298,  E),  just  like 
the  spores  of  the  bryophytes  and  pteridophytes,  and  their  further 
history  shows  them  to  correspond  exactly  to  the  microspores.1 
The  pollen  sac  is  then  a  microsporangium,  and  the  stamen  a 
microsporophyll.  The  pollen  sacs  develop  from  a  group  or  region 
of  cells  as  in  the  horsetails,  lycopods,  and  Selaginella,  and  not 
from  a  single  surface  cell  as  in  the  common  ferns. 

The  pollen  grains  are  produced  in  enormous  quantities,  and 
being  set  free  by  the  splitting  of  the  pollen  sacs,  they  are  scat- 
tered as  fine  yellow  dust  by  the  wind.  Sometimes  pollen  is  carried 
from  pine  forests  by  the  wind  for  many  mile^,  falling  as  so-called 
showers  of  sulphur.  The  pollen  grains  are  especially  adapted 
for  distribution  by  the  wind,  for  the  outer  layer  of  the  cell  wall 
is  swollen  on  two  sides.to  form  outstanding  wings  (Fig.  298,  F). 

1  This  relationship  is  further  established  by  the  count  of  the  chromosomes 
in  the  Scotch  pine  (Pinua^ylvestris),  which  shows  that  the  pollen  grain  has 
12,  while  certain  tissues  of  the  pine  sporophyte  have  24.  Pollen  formation 
is  then  the  period  of  chromosome  reduction  when  the  sporophyte  generation 
passes  over  to  the  gamptophyte,  as  explained  in  Sees.  334  and  335.  Similar 
chromosome  reduction  undoubtedly  takes  place  with  the  formation  of  the 
embryo  sac  in  the  nucellus'. 


FIG.  299.   Carpellate  cone,  carpels,  and  seed  of  the  Scotch  pine 
(Pinus  sylvestris) 

A,  young  growth  with  carpellate  cones,  about  three  weeks  after  the  opening  of 
the  terminal  bud :  n,  young  pine  needles.  7>,  inner  and  side  view  of  a  cone 
scale  at  the  time  of  pollination  as  shown  in  A :  b,  bract ;  o,  ovules.  C,  inner 
and  side  view  of  scales  from  a  two-year-old  cone  as  shown  in  D:  b,  bract; 
o,  fertilized  ovules  now  rapidly  maturing  into  winged  seeds ;  w,  the  developing 
wings.  D,  a  two-year-old  cone.  E,  a  mature  winged  seed.  F,  section  of 
mature  seed :  t,  hard  seed  coat,  or  testa,  developed  from  the  integument  of  the 
ovule  (see  Fig.  300,  A,  i) ;  n,  a  membranous  seed  coat  which  is  the  remains  of 
the  nucellus  (see  Fig.. 300,  A'ri) ;  en,  endosperm  or  tissue  of  the  female  gameto- 
phyte  (see  Fig.  300,  A) ;  era,  e.nbryo  with  group  of  cotyledons  c  and  the 
suspensor  s ;  m,  micropylar  end  of  seed 

370 


THE  CARPELLATE   CONE  371 

The  carpellate  cone.  The  carpellate  cones  have  a  complex 
structure  that  cannot  here  be  described  in  detail.  They  are 
borne  singly  or  in  groups  of  two  or  three  at  the 'ends  of  the  new 
growth  in  the  spring  (Fig.  299,  A)  simultaneously  with  the 
staminate  cones.  Each  cone  is  composed  of  scales  arranged 
somewhat  spirally.  Each  scale  (Fig.  299,  B)  is  believed  to  be 
a  group  of  three  fused  carpels  (the  point  representing  a  sterile 
carpel  between  two  fertile  ones).  The  scale  bears  a  pair  of 
ovules  below  on  the  inner  face,  near  the  place  where  it  is 
attached  to  the  axis  of  the  cone. 

The  ovule  has  a  large  nucellus,  surrounded  by  an  integu- 
ment, which  bears  two  appendages  looking  like  a  pair  of  horns 
in  miniature  (Fig.  299,  B,  o).  The  embryo  sac  which  develops 
in  the  center  of  the  nucellus  is  one  of  a  group  of  four  cells,  or 
tetrad,  which  shows  its  relationship  to  a  spore  (megaspore)  and 
to  the  pollen  grain.  The  other  three  cells  of  the  tetrad  fail  to 
develop,  so  that  all  the  strength  of  the  ovule  is  given  to  this 
single  functional  megaspore  which  produces  the  female  gameto- 
phyte.  The  ovule  is  an  outgrowth  from  the  surface  of  the  car- 
.pel,  its  nucellus  (Fig.  300  A,  n)  corresponds  to  a  megasporangium, 
and  the  integuments  (Fig.  300,  A,i)  are  probably  protective 
investments.  The  integuments  do  not  completely  inclose 
the  nucellus,  but  there  is  left  a  small  opening  at  the  tip 
(Fig.  300,  A,  m)  called  the  micropyle. 

353.  Pollination  in  the  pine.  The  young  carpellate  cones  are 
upright  when  they  first  appear,  and  the  scales  are  slightly  sepa- 
rated from  one  another.  When  the  pollen  is  shed  in  clouds 
from  the  stamens  some  of  the  grains  are  carried  by  the  wind 
to  the  carpellate  cones  and  sift  in  between  the  scales,  collecting 
in  little  drifts  near  the  ovules.  This  is  the  process  of  pollination. 
At  this  time  there  are  globules  of  moisture  between  the  two  horn- 
like appendages  of  the  ovules,  and  the  pollen  grains  are  caught 
by  these.  The  fluid  gradually  dries  up,  drawing  the  pollen  grains 
toward  the  micropyle,  and  finally  into  a  cavity'ealled  the  pollen 
chamber  (Fig.  300,  A,  pc),  which  lies  just  above  the  nucellus. 


372 


THE  SPERMATOPHYTES 


Meanwhile  the  scales  of  the  cone  close  together  and  the  cone 
bends  over  until  it  hangs  downward.  This  is  a  curious  behavior, 
although  there  is  evident  advantage  to  the  plant,  for  the  cone  is  now 
in  a  better  position  to  protect  the  ovules  from  rain  or  dust  which 
might  enter  between  the  scales  if  the  cones  remained  upright. 


B 


FIG.  300.  "The  gametophytes  of  the  pine 

,  diagram  of  a  section  of  a  year-old  ovule :  embryo  sac  with  mature  archegonia 
ar  imbedded  in  the  tissue  of  the  endosperm  (female  gametophyte) ;  pollen 
tubes  (male  gametophytes)  growing  down  through  the  tissue  of  the  nucellus 
nj  pc,  pollen  chamber;  m,  micropyle;  i,  integument.  Z>,  germinating  pollen 
grain,  showing  young  male  gametophyte :  t,  tube  nucleus ;  ff,  generative  nucleus ; 
p,  prothallial  cell.  C,  tip  of  pollen  tube  applied  to  the  egg:  t,  tube  nucleus; 
s,  the  two  sperm  nuclei.  7),  a  mature  archegonium  sunken  in  the  tissue  of  the 
endosperm,  showing  the  large  egg  surrounded  by  a  jacket  of  cells  rich  in  proto- 
plasm :  two  neck  cells  of  the  archegonium  shown  just  above  the  egg.  —  /?,  C', 
after  Miss  Ferguson 


THE  GAMETOPHYTES  OF  THE  PINE  373 

354.  The  gametophytes  of  the  pine.  The  pine,  like  all  seed 
plants,  is  of  course  lieterosporous  because  it  has  microspores 
(pollen  grains)  and  megaspores  (embryo  sacs) ;  so  there  are  two 
gametophytes,  male  and  female. 

The  male  gametophyte.  The  male  gametophyte,  as  in  most,  if 
not  all,  seed  plants,  begins  to  develop  before  the  pollen  is  shed. 
There  are  three  nuclear  divisions  .which  cut  off  two  small  cells, 
called  prothallial  cells,  of  which  traces  may  sometimes  be  found 
against  the  wall  of  the  pollen  grain  (Figs.  298,  F,  p ;  300,  B,p}. 
The  third  division  leaves  the  pollen  grain  with  a  nucleus  (the 
tube  nucleus)  in  the  central  region  and  a  small  lens-shaped  cell 
(the  generative  cell)  at  one  side  (Fig.  298.  F,  g).  This  is  the 
condition  when  the  pollen  is  shed. 

Shortly  after  pollination  the  pollen  tubes  begin  to  develop 
in  the  pollen  chamber  (Fig.  300,  A,p  c)pbut  their  development 
is  very  slow  until  the  following  spring.  Then  the  large  tube 
nucleus  passes  to  the  tip  of  the  tube,  which  grows  rapidly 
towards  the  center  of  the  nucellus  (disorganizing  the  surround- 
ing tissue  as  it  does  so),  where  the  female  gametophyte  lies 
within  the  embryo  sac.  The  generative  cell  now  divides  into  a 
stalk  and  body  cell  which  pass  into  the  tube.  The  body  cell 
forms  two  sperm  nuclei  a  few  weeks  later.  Four  nuclei  are 
then  finally  present  at  the  end  of  the  pollen  tube  (two  sperm 
nuclei,  the  tube  nucleus,  and  that  of  the  stalk  cell).  The  pollen 
tube  has  now  reached  the  embryo  sac  and  is  ready  to  discharge 
its  contents  into  one  of  the  eggs  developed  by  the  gametophyte 
(Fig.  300,  D). 

The  female  gametophyte.  The  embryo  sac  (megaspore)  is  a 
one-nucleate  cell  at  about  the  time  of  pollination.  This  nucleus 
gives  rise  by  repeated  divisions  to  a  large  number  of  nuclei  that 
lie  at  first  freely  in  the  protoplasm  as  the  embryo  sac  gradually 
increases  in  size.  Later,  cell  walls  are  formed  around  the  free 
nuclei,  and  the  entire  embryo  sac  becomes  filled  with  a  delicate 
tissue  called  the  endosperm  (Fig.  300,  A),  which  corresponds  to 
the  vegetative  portion  of  a  prothallium.  It  takes  almost  a  full 


374  THE  SPERMATOPHYTES 

year  for  the  female  gametophyte  to  reach  this  stage  of  devel- 
opment, when  it  occupies  the  greater  part  of  the  nucellus.  In 
the  spring  following  the  pollination  of  the  cone,  the  endosperm 
forms  a  group  of  several  archegonia  at  its  micropylar  end.  Each 
archegonium  (Fig.  300,  D)  consists  of  a  much-reduced  neck 
region,  generally  composed  of  four  cells,  and  the  very  large 
egg  which  lies  imbedded  in  the  endosperm,  whose  cells  form  an 
investment  around  it  called  the  jacket.  The  egg  is  filled  with 
dense  protoplasm  and  contains  much  food  material  supplied 
through  the  cells  of  the  jacket. 

This  is  the  condition  of  the  female  gametophyte  thirteen 
months  after  pollination.  At  about  this  time  the  pollen  tube 
reaches  the  embryo  sac  and  entering  it  passes  between  the 
neck  cells  of  an  archegonium,  where  its  tip  fuses  with  the  egg 
membrane.  The  contents  at  the  end  of  the  pollen  tube  are  dis- 
charged into  the  egg,  including  not  only  the  two  sperm  nuclei, 
but'  also  the  tube  nucleus  and  that  of  the  stalk  cell.  One  of 
the  sperm  nuclei  moves  towards  the  egg  nucleus,  which  lies 
near  the  center  of  the  egg,  and  fusing  with  it  completes  the 
act  of  fertilization.  The  other  three  nuclei  break  down  and 
soon  disappear. 

355.  The  development  of  the  embryo  in  the  pine.  Fertiliza- 
tion takes  place,  as  described  above,  a  little  more  than  a  year 
after  pollination.  The  cone  during  this  time  has  increased 
greatly  in  size,  but  is  generally  hardly  a  third  as  large  as  the 
mature  seed-bearing  cone. 

The  fertilized  egg  soon  begins  to  develop  the  pine  embryo. 
This  is  a  complicated  history,  which  cannot  be  described  here  in 
detail.  The  embryo  is  however  formed  at  the  end  of  a  structure 
called  the  suspensor  (Fig.  299,  F,  s),  whose  development  carries 
the  embryo  into  the  center  of  the  endosperm,  where  it  lies 
in  a  favorable  situation  for  its  nourishment.  The  embryo 
(Fig.  299,  F,  em)  is  straight,  and  the  stem  part  is  surrounded 
by  a  circle  of  seed  leaves  called  cotyledons.  The  pine  seedling 
is  shown  in  Fig.  12. 


THE  LIFE  HISTORY   OF  A  GYMNOSPERM          375 

Meanwhile  the  integument  becomes  firmer  and  finally  forms 
the  hard,  protective  seed  coat,  or  testa  (Fig.  299,  F,  t).  Adjacent 
tissue  of  the  cone  scale  above  the  ovule  develops  a  membranous 
wing  (Fig.  299,  C,  w),  which  separates  from  the  scale  of  the  cone 
with  the  ovule  as  a  part  of  the  seed.  It  takes  another  full  year 
for  these  changes  to  take  place,  and  the  cone  is  not  fully  mature 
(Fig.  299,  D)  and  the  seeds  ripe  until  somewhat  more  than  two 
years  after  pollination.  Then  the  scales  of  the  cone,  now  quite 
woody  in  texture,  separate,  and  the  seeds  are  shaken  out,  and 
since  they  are  winged  (Fig.  299,  .Z?)  they  may  be  carried  for  a 
considerable  distance  by  the  wind. 

356.  The  life  history  of  a  gymnosperm.  The  life  history  of 
a  gymnosperm,  beginning  with  the  sporophyte  (for  the  gameto- 
phyte  phases  are  now  so  inconspicuous  that  they  only  appear 
during  the  process  of  seed  formation),  may  be  formulated  as 
follows  : 

pollen  grain  —  Male  Gametophyte—  sperm  nucleus  , 

SpOTO-  (<-,,,„„,,) 


P  y          embryo  sac  —  Female  Gametophyte  —  egg 


(megasporc)  (protoplasmic  contents 

of  embryo  sac) 


This  in  abbreviated  form  becomes 
.p  a  —  M  G  —  s 


v    n  —  $  etc- 

es  —  te  G  —  e  -^ 

This  formula  should  be  compared  with  that  of  some  hetero- 
sporous  pteridophyte,  as  Selaginella  (Sec.  326),  to  make  clear  the 
relationships.  When  carefully  studied  it  will  be  found  to  be 
merely  an  elaborated  form  of  the  simple  formula  of  alternation 
of  generations. 

G<Se>-S-sp-G<Se>-S~sp-G,  etc. 

The  peculiarities  of  the  life  history  of  a  gymnosperm  are  due 
to  heterospory  (and  this  is  true  of  all  seed  plants),  by  means  of 
which  two  sexual  plants,  male  and  female,  have  been  differen- 
tiated, and  the  fact  that  both  gametophytes  live  wholly  or 
almost  wholly  as  parasites  upon  the  sporophyte. 


376  THE   SPERMATOPHYTES 

SUBDIVISION  II.    THE  ANGIOSPEEMS,  OK 
ANGIOSPERMS 

357.  The  angiosperms.    The  angiosperms  (meaning  seeds  in 
a  vessel)  are  distinguished  by  the  fact  that  the  ovules  are  devel- 
oped in  a  closed  case  (ovule  case  or  ovary)  formed  by  .the  car- 
pels, sometimes  alone  but  often  together  with  adjacent  tissue  of 
the  stem.    This  immense  assemblage  of  plants,  with  more  than 
120,000  species,  forms  the  greater  part  of  the  earth's  vegetation 
and  includes  the  most  successful  groups,  dominating  most  of 
the  land  floras.    It  is  a  much  more  varied  assemblage  than  the 
gymnosperms,  and  successful  in  every  vegetation  form  (herb, 
shrub,  or  tree).    The  angiosperms  adapt  themselves  to  all  sorts 
of  life  conditions,  some  of  them  being  aquatics,  others  covering 
the  meadows,  prairies,  and  heaths,  certain  groups  entering  the 
deserts,  and  the  trees  forming  forests  generally  accompanied 
by  undergrowths  of  shrubs.    They  occupy  the  highest  points  of 
plant  evolution,  but  along  a  great  many  very  divergent  lines,  for 
some  of  the  culminating  groups  are  the  grasses,  the  hardwood 
trees,  the  composite  groups,  the  orchids,  etc. 

The  general  structure  of  the  angiosperms,  including  the  roots, 
stems,  leaves,  flowers,  and  fruits,  together  with  many  principles 
of  plant  physiology  best  illustrated  in  this  group,  have  been 
described  in  Part  I.  This  account  will  consider  chiefly  the  life 
history,  with  especial  reference  to  the  gametophyte  generations 
and  significance  of  the  flower. 

358.  The   angiosperm  flower.    The  essential   structures  of 
the  angiosperm  flower  (Fig.  301),  as  of  the  gymnosperms,  are 
the   stamens   (microsporophylls)   and  the  carpels  (megasporo- 
phylls) ;  but  in  addition  to  these  some  accessory  parts  are  gen- 
erally present,  which  are  either  modified  leaves  of  the  plant, 
or  sometimes   stamens  and  carpels  that  have  become  sterile. 
These   accessory  parts    constitute   the  perianth  (Fig.  301,  p), 
situated  on  the  stem  just  below  the  stamens  and  carpels,  and 
are  generally  showy  structures,  but  also  protective,  at  leact  in 


THE  ANGIOSPERM  FLOWER 


377 


the  bud.  The  perianth,  as  a  rule,  gives  the  characters  of  color 
and  form  which  in  popular  usage  define  a  flower.  It  is  a  very 
important  accession,  for  it  has 
resulted  in  some  remarkable 
adaptations  and  devices  on  the 
part  of  the  plant  to  insure  pol- 
lination by  the  visits  of  insects 
(see  Chapter  xxxn).  The  struc- 
ture of  the  perianth,  with  its 
parts,  —  sepals  and  petals,  — 
is  described  in  Chapter  xm. 
Besides  having  the  perianth, 
the  angiosperm  flower  is  pecul- 
iar in  that  the  ovules  are  not 
normally  exposed  on  the  sur- 
face of  the  carpels.  This  means 
that  the  carpels,  either  singly 
or  in  groups,  form  closed  struc- 
tures, which  may  be  termed 
ovule  cases.  The  ovule  case, 
generally  called  the  ovary  (an 
unfortunate  term,  for  it  does 
not  produce  eggs  but  ovules), 
bears  a  receptive  surface, 
termed  the  stigma,  upon  which 
the  pollen  grains  may  ger- 
minate. The  stigma  may  be 
raised  upon  a  stalk,  or  style. 


FIG.  301.  The  lily  (Lilium 
Philadelphia  um} 


Ovule -case,  Style,  and  Stigma    ^dissected  flower,  showing  the  pistil  and 

constitute  the  pistil  (meaning 
a  pestle),  which  is  said  to  be 
simple  when  only  a  single  car- 
pel is  involved,  and  compound 
if  there  is  a  group  of  carpels. 
The  various  arrangements  of 


stamens :  p,  parts  of  the  perianth  which 
have  been  cut  away ;  s,  bases  of  stamens 
cut  off.  B,  floral  diagram :  p,  perianth, 
composed  of  two  circles  of  similar  and 
petal-like  parts;  s,  stamens,  likewise  in 
two  circles ;  section  of  ovule  case  (ovary) 
shown  in  the  center,  composed  of  three 
carpels  (c)  so  united  as  to  form  three 
locules  containing  the  ovules 


378 


THE   SPERMATOPHYTES 


the  carpels  to  form  different  types  of  pistils  are  described  in 
Sees.  156  and  157. 

Another  characteristic  of  the  angiosperm  is  the  production 
of  fruit.    A  fruit  is  a  ripened  ovule  case,  or  ovary,  frequently 


FIG.  302.    Anther  and  pollen  of  the  lily 

A,  mature  anther,  showing  the  four  locules,  or  chambers,  containing  pollen  grains : 
the  anther  opens  lengthwise  on  both  sides  along  the  lines  of  cells  shown  at  x. 
B,  stages  in  the  formation  of  pollen  grains  in  a  group  of  four  (tetrad)  within 
the  pollen  mother  cell.  (7,  mature  pollen  grain  with  early  stages  in  the  devel- 
opment of  the  male  gametophyte :  t,  tube  nucleus ;  g,  generative  nucleus 


THE  STAMEN  AND  THE   FORMATION  OF  POLLEN     379 

with  accessory  parts.  The  gymnosperms  do  not  have  the  exact 
equivalents  of  fruits,  although  the  berry-like  structures  of  the 
yew  appear  at  first  glance  to  be  similar  and  the  cone  is,  of 
course,  a  protective  structure  for  the  seeds.  True  fruits,  as  the 
term  is  used  when  applied  to  the  angiosperms,  are  seed  cases  of 
various  forms,  —  structures  which  are  sometimes  merely  protec- 
tive, and  sometimes  fleshy  and  attractive  to  animals  for  food. 
They  are  described  in  Chapter  xvi. 

The  pistil  distinguishes  the  angiosperms  from  the  gymno- 
sperms, and  is  a  more  important  feature  of  the  angiosperm 
flower  than  the  perianth,  which  is  frequently  inconspicuous, 
and  sometimes  wholly  or  almost  wholly  absent.  But  the  pistil 
in  combination  with  a  showy  perianth  of  some  peculiar  and 
specialized  form  gives  the  highest  types  of  flower  structure. 
The  most  important  of  these  are  discussed  in  Chapter  xm. 
This  account  will  only  describe  the  stamens  and  carpels  in  their 
functions  as  spore-producing  organs  developing  microspores 
(pollen)  and  megaspores  (embryo  sacs). 

359.  The  stamen  and  the  formation  of  pollen.  The  parts 
of  a  stamen  are  described  in  Sees.  171  to  173.  Pollen  formation 
takes  place  generally  in  four  regions  of  the  anther,  which  become 
pollen  sacs,  or  loculcs  (Fig.  302,  A).  The  cells  of  these  regions 
develop  the  pollen  grains  in  groups  of  four,  or  tetrads  (Fig.  302,7?), 
and  are  consequently  pollen  mother  cells.  This  process  is  iden- 
tical with  that  of  spore  formation  in  the  pteridophytes  and 
bryophytes.1  The  pollen  mother  cell  is  a  spore  mother  cell,  and 
the  pollen  grain  a  spore,  or  more  exactly  a  microspore. 

The  pollen  sacs  are  .  sporangia,  and  like  the  sporangia  of 
the  horsetails,  lycopods,  Selaginella,  and  the  pollen  sacs  of  the 

1  As  in  the  case  of  the  gymnosperms,  the  count  of  the  chromosomes  during 
pollen  formation  shows  it  to  be  a  period  of  chromosome  reduction,  when 
the  sporophyte  generation  passes  over  to  the  gametophyte,  as  explained  in 
Sees.  334  and  335.  Thus  24  chromosomes  have  been  counted  in  various  tissues 
of  the  lily  plant,  but  only  12  appear  in  the  nuclear  divisions  in  the  pollen 
mother  cell  (Fig.  302,  B).  These  cells,  it  may  be  remarked,  are  exceedingly 
good  subjects  for  the  study -of  nuclear  division 


380 


THE   SPERMATOPHYTES 


gymnosperms  (Sec.  352), they  develop  from  a  group  or  large  region 
of  cells,  and  not  from  a  single  surface  cell  as  in  the  sporangium 
of  the  common  ferns.  The  pollen  sacs  open  along  certain  lines 
(Fig.  302,  x)  or  by  pores,  and  the  pollen  is  thus  set  free.  The 
pollen  is  carried  in  various  ways  to  the  stigma  of  the  pistil,  as 
described  in  Chapter  xxxn,  and  its  application  to  this  structure 


FIG.  303.   Section  of  the  ovule  case  (ovary)  of  the  lily 

Diagram  of  a  cross  section  of  a  young  ovule  case,  showing  the  three  carpels  c ;  each 
young  ovule  o  has  a  large  embryo  sac  mother  cell  e  within  the  small  nucellus 
n,  and  shows  the  developing  inner  and  outer  integuments  ii  and  oi 

constitutes  pollination  in  the  angiosperms.  Wind,  direct  con- 
tact of  the  anthers  with  the  stigma,  or  the  visits  of  insects  are 
means  by  which  pollination  is  effected  in  this  group  of  plants. 

360.  The  carpel  and  the  formation  of  the  ovule.  The  ovules 
are  developed  as  outgrowths  from  the  surface  of  the  carpels 
(Fig.  303),  or  in  some  cases  from  regions  of  the  stem,  when  this 


THE  GAMETOPHYTES  OF  AN  ANGIOSPERM 


381 


structure  enters  into  the  formation  of  the  ovule  case.  Each 
ovule  consists  of  a  central  region  called  the  nucellus  (Figs.  303; 
306,  A  ;  309,  A,  n),  which  becomes  en- 
veloped by  two  protective  integuments 
(Figs.  303  ;  306,  A  ;  309,  A,  B}  C,  ii,  oi) 
that  arise  from  its  base  and  grow  up 
around  it,  forming  a  small  opening 
above  termed  the  micropyle  (meaning 
little  gate).  A  cell  in  the  interior  of 
the  nucellus  becomes  the  embryo  sac 
(Figs.  303,  e;  306,  A,  B),  which  in 
most  cases  is  the  exact  equivalent  of 
a  megaspore.  This  is  proved  by  the 
fact  that  the  embryo  sac  in  such  forms 
is  one  of  a  group  of  four  .cells,  or  tetrad 
(Fig.  304),  and  that  the  development 
of  this  group  follows  the  same  history 
as  in  pollen  and  spore  formation.  The 
nucellus  is  therefore  a  megasporangium. 
Certain  forms  of  angiosperms,  as  the  FIG.  304.  A  group  of  four 
lily,  have  given  up  the  formation  of  inegaspores  (tetrad)  in  the 
tetrads,  and  the  spore  mother  cell  de- 
velops directly  into  the  embryo  sac.1 

361.  The  gametophytes  of  an  an- 
giosperm.  The  male  gametophyte 
(contents  of  the  pollen  grain  and  tube) 
is  clearly  similar  to  that  of  the  gymno- 
sperm ;  but  the  female  gametophyte  of  the  angiosperm  is  a 
very  much  more  reduced  structure  than  anything  in  the  gym- 
nosperms. 

1  In  these  cases  the  first  two  nuclear  divisions  within  the  embryo  sac 
have  the  peculiarities  of  those  in  all  spore  mother  cells.  In  the  lily  the 
nuclei  of  the  nucellus  have  24  chromosomes,  but  the  nuclei  of  the  embryo 
sac  have  12.  This  shows  that  the  two  nuclear  divisions  characteristic  of 
spore  formation  have  become  a  part  of  the  gametophyte  phase  of  the  plant's 
life  histoiy. 


nucellus  of  an  ovule 
(Canna) 

The  upper  three  megaspores 
of  the  group  are  breaking 
down,  while  the  lower  is 
rapidly  enlarging  to  become 
the  embryo  sac.  —  After 
Wiegand 


382 


THE  SPERMATOPHYTES 


The  male  gametophyte.  As  in  the  gymnosperms,  the  male 
gametophyte  begins  its  history  in  the  pollen  grain  before  the  lat- 
ter has  been  shed.  The  first  division  forms  a  tube  nucleus  and  a 
generative  cell  (Fig.  302,  C).  The  nucleus  of  the  generative  cell 
divides  sooner  or  later  to  form  two  sperm  nuclei.  These  three 
nuclei,  with  the  rest  of  the  protoplasm,  constitute  all  there  is  of 

the  male  gametophyte  (Fig.  305). 
The  pollen  grains  germinate  on 
the  stigma  of  the  pistil,  finding 
there  suitable  fluids  to  start  their 
growth.  Each  puts  forth  a  tube 
(Fig.  156,  A,  B,  C)  which  pene- 
trates the  stigma  and  grows  down- 
ward toward  the  ovule  case 
(ovary).  The  tube  nucleus  and  the 
generative  cell  (or  the  two  sperm 

nuclei  if   already  formed)  enter 
FIG.  305.  Pollen  grain  of  the  elder    the  tube  and?  passing  to  the  tip> 

accompany  its  growth  (Fig.  156, 


(Sambucus) 


The  two  sperm  cells  s  and  the  tube  ^  ™  ^ 
nucleus  t,  with  the  remaining  pro-  '  '  /' 
toplasm,  constitute  the  entire  male  The  pollen  tubes  grow  through 

the  tissues    of    the    stigma   and 

style  (if  present)  frequently  over  definite  paths  and  enter  the 
micropyles  of  the  ovules.  This  behavior  resembles  the  way  in 
which  parasitic  fungi  grow  through  the  tissues  of  their  hosts, 
and  it  is  clear  that  the  pollen  tubes  live  largely  or  wholly  para- 
sitically  on  the  sporophyte.  On  entering  an  ovule  the  pollen 
tube  penetrates  the  nucellus  and  grows  toward  the  embryo 
sac,  which  by  this  time  has  developed  the  female  gameto- 
phyte. 

The  female  gametophyte.  The  mature  female  gametophyte 
of  an  angiosperm  (Fig.  306,  B)  contains  only  eight  nuclei,  the 
products  of  three  nuclear  divisions  in  the  embryo  sac.  These 
are  distributed  as  follows  :  There  is  a  group  of  three  nuclei  at 
the  micropylar  end  of  the  embryo  sac  (Fig.  306,  B,  m),  forming  the 


FERTILIZATION  AND  DOUBLE  FERTILIZATION      383 

egg  apparatus,  of  which  one,  with  surrounding  protoplasm,  con- 
stitutes the  egg,  and  the  other  two  are  called  synergids  (meaning 
co-workers).  There  is  a  group  of  three  nuclei  at  the  opposite 
end  of  the  sac,  called  antipodal  nuclei  (Fig.  306,  B,ant],  which 
frequently  become  inclosed  by  delicate  walls  and  possibly  repre- 
sent a  prothallial  region.  The  remaining  two  nuclei,  called  polar 


FIG.  306.   The  ovule  and  embryo  sac  of  the  lily 

A,  ovule  with  mature  embryo  sac:  the  inner  integument  ii  has  grown  beyond  the 
nucellus  n;  oi,  outer  integument;  m,  micropyle.  JB,  mature  embryo  sac:  egg 
apparatus  at  the  micropylar  end  m ;  e,  egg;  s,  synergids ;  the  two  polar  nuclei 
p  are  about  ready  to  fuse  near  the  center  of  the  sac ;  ant,  antipodal  nuclei 

nuclei  (Fig.  306,  B,  p),  pass  from  the  opposite  ends  to  the  center 
of  the  embryo  sac,  where  they  later  unite. 

362.  Fertilization  and  double  fertilization.  The  tip  of  the 
pollen  tube  fuses  with  the  end  of  the  embryo  sac,  near  the 
synergids,  and  the  two  sperm  nuclei  are  discharged  into  the  sac. 
The  tube  nucleus  has  generally  broken  down  and  disappeared 
entirely  by  this  time  (Fig.  156,  F,  G).  One  of  the  sperm  nuclei 
unites  with  the  egg  nucleus  (Fig.  307,  e,  fs),  and  this  is  the 
process  of  fertilization. 


384 


THE  SPERM ATOPHYTES 


The  other  sperm  nu- 
cleus is  known  in  a 
number  of  forms  to  pass 
to  the  center  of  the  sac 
and  unite  with  the  two 
polar  nuclei,  constitut- 
ing a  triple  fusion  (Fig. 
307,  p,  p,  ss),  and  form- 
ing a  large  nucleus, 
called  the  endosperm 
nucleus.  Since  the  en- 
dosperm nucleus  has 
an  important  history  in 
the  development  of  the 
seed,  this  peculiar  behav- 
ior of  the  second  sperm 
nucleus  is  important,  and 
it  is  called  the  double 
fertilization  of  the  em- 
bryo sac.1 

1  Double  fertilization  is 
probably  the  explanation  of 
the  phenomenon  called  xenia, 
which  is  the  appearance  at 
once  in  the  seed  of  some 
character  of  the  male  parent. 
Thus  a  yellow  or  white  kind 
of  corn,  when  pollinated  from 
a  blue  or  red  variety,  will 
produce  blue  or  red  kernels. 

This  color  in  the  corn  is  pres- 
The  nrst  sperm  nucleus  fa  fusing  with  the  egg         .    .      ,,      endosi)erm    and 
nucleus  e ;  the  second  sperm  nucleus  ss  is  fusing  ^ospei  m,   am 

with  the  two  polar  nuclei  p  near  the  center  of  the  character  comes  into  the 
the  sac,  constituting  the  so-called  double  ferti-  seed  through  the  second 
lization ;  p,  pollen  tube ;  s,  synergid  breaking  sperm  nucleus.  For  an  ac- 
down;  ant,  antipodals;  ii,  inner  integument;  count  of  xenia  see  Webber, 
m,  micropyle  "Xenia,  or  the  Immediate 

Effect  of   Pollen  in  Maize,"   United   States  Department  of   Agriculture, 
Division  of  Vegetable  Physiology  and  Pathology,  Bulletin  22,  1900. 


FIG.  307.   Fertilization  in  the  embryo  sac 
of  the  lily 


DEVELOPMENT  OF  THE  EMBRYO  AND  ENDOSPERM  385 


363.  The  development  of  the  embryo  and  endosperm. 

fertilized  egg  develops  the  em- 
bryo, but  as  in  gymnosperms, 
there  is  generally  a  preliminary 
growth  called  the  suspensor 
(Fig.  309,  D,  E,  H,  s),  which  car- 
ries the  young  embryo  into  the 
center  of  the  sac.  The  endo- 
sperm nucleus  begins  to  divide 
at  once  after  its  formation,  by 
the  triple  fusion  of  the  second 
sperm  nucleus  with  the  two  po- 
lar nuclei  (Fig.  307).  It  gives 
rise  to  a  large  number  of  nuclei, 
which  become  distributed  in  the 
protoplasm  of  the  rapidly  enlar- 
ging embryo  sac  (Figs.  308,  e  ; 
309,  H,  e).  Later,  walls  begin  to 
form  around  these  endosperm 
nuclei,  first  in  the  outer  regions 
of  the  embryo  sac,  and  finally 
the  whole  sac  becomes  filled 
with  a  delicate  tissue. 

This  tissue  is  called  the  endo- 
sperm, and  the  embryo  becomes 
imbedded  within  it  as  in  the  pine. 
But  this  endosperm  has,  of  course, 
a  very  different  origin  from  that 
Of  the  gymnosperms,  and  is  a 
special  development  peculiar  to 


The 


•  308.  Development  of  the  embryo 

and  the  endosperm  of  the  lily 


,          f    .  m.  The   embryo  em  has  developed   from 

the  anglOSpermS.     The  group  of       the    fertilized    egg;    e,    endosperm 

antipodal  cells   possibly   corre-     nuclei  which  have  been  derived  fr°m 

f  •  .  the  triple-fusion  nucleus,  —  that  is, 

Sponds  to  the  endosperm  in  the       the  two  polar  nuclei  united  with  the 

gymnosperms,  and  the  egg  ap- 

&J 

paratus  has  been  regarded  as  a 


!f.conQ(JJperm  nucleu.s 

Fig.  307)  ;   ii,  inner  integument  ;   m, 

micropyie 


FIG.  309.    Development  of  the  ovule  and  embryo  of  the  shepherd's 
purse  (Capsella) 

A,  young  ovule,  showing  origin  of  two  integuments  at  base  of  nucellus  n.  B,  outer 
integument  growing  beyond  the  inner,  and  the  ovule  beginning  to  bend  over: 
es,  embryo  sac.  C,  diagram  of  a  later  stage  with  mature  embryo  sac.  D,  devel- 
opment of  the  suspensor  s.  E,  early  divisions  of  the  terminal  cell  (embryo 
cell).  F,  later  stage,  showing  the  differentiation  of  an  outer  cell  layer  in  the 
embryo,  which  is  to  become  the  epidermis.  G,  the  two  cotyledons  c  and  the 
root  region  r  now  clearly  defined.  H,  lengthwise  section  of  an  ovule,  show- 
ing the  position  of  an  embryo  in  an  embryo  sac :  em,  embryo ;  s,  suspensor ;  e, 
endosperm;  U,  inner  integuments;  oi,  outer  integument;  m,  micropyle.  —  A, 
B,  C,  adapted  after  Campbell 

386 


THE  DEVELOPMENT  OF  THE  FLOWER 


387 


reduced  archegonium.  However,  it  is  possible  that  all  three  of 
the  nuclei  in  the  egg  apparatus  represent  eggs,  only  one  of 
which  is  functional. 

While  the  embryo  and  endosperm  are  developing,  the  ovule 

increases  greatly  in  size,  and  its  integuments  change  into  the 

s, 


E  D 

FIG.  310.   Development  of  the  flower  of  the  shepherd's  purse  (Capsella) 

A,  tip  of  stem,  showing  the  origin  of  the  flowers:  s,  first  appearance  of  the  sepals 
in  the  flower  /.  B,  sepals  well  along  in  their  development  and  stamens  st 
appearing.  6',  later  stage,  showing  the  two  young  carpels  c  and  the  beginnings 
of  the  petals  p.  D,  later  stage  lettered  as  in  the  preceding.  E,  the  petals  now 
well  developed,  and  the  ovules  beginning  to  arise  on  the  inner  face  of  the 
carpels,  not  yet  united  above  to  form  the  closed  pistil 

seed  coats.  In  some  plants,  as  in  the  squash  (Fig.  1),  peas,  and 
beans,  the  embryo  finally  fills  the  entire  seed,  and  the  endo- 
sperm is  almost  completely  crowded  out,  being  represented  by  a 
thin  membrane  around  the  embryo.  In  other  forms,  as  the  corn 
(Fig.  3),  asparagus,  and  poppy  (Fig.  4),  the  embryos  remain  small, 
and  the  endosperm  is  conspicuous  as  a  tissue  richly  stored  with 
food  material. 


388  THE  SPERMATOPHYTES 

364.  The  development  of  the  flower.  The  development  of  the 
parts  of  a  flower  would  be  expected  to  progress  in  the  order  of 
sepals,  petals,  stamens,  and  carpels,  for  this  is,  of  course,  the 
order  of  their  position  on  the  flower  stalk,  beginning  from  below. 
The  parts  of  many  flowers  do  arise  in  this  order,  but  there  are 
often  irregularities  due  to  the  delayed  appearance  of  some  organs. 
For  example,  in  the  shepherd's  purse  the  petals  are  formed  last, 
arising  between  the  sepals  and  carpels  when  the  latter  are  far 
along  in  their  development  (Fig.  310,  C,p). 

The  carpels  are  clearly  separate  in  the  beginning  (Fig.  310, 
C,  D,  c),  and  the  ovules  at  first  may  be  exposed  on  their  surface 
(Fig.  310,  E),  but  sooner  or  later  the  carpels  unite  above,  so  that 
the  ovules  are  finally  contained  in  the  ovule  case  (ovary). 

A  study  of  flower  development  makes  clear  the  significance  of 
perigyny  and  epigyny  (Sec.  157),  for  it  shows  that  the  apparent 
fusion  of  parts,  frequently  called  coalescence,  when  sepals,  petals, 
or  stamens  seem  to  be  united  to  one  another  or  to  the  carpels 
(see  diagrams,  Fig.  136),  is  due  to  the  formation  of  tubular  out- 
growths from  zones  of  tissue  below  the  floral  parts.  The  parts 
which  are  most  frequently  affected  by  these  zonal  outgrowths 
are  the  carpels,  and  it  seems  probable  that  the  compound  pistil 
may  have  arisen  from  their  activities.  In  many  cases  the  ovules 
are  developed  from  tissue  that  is  probably  really  a  part  of  the 
tip  of  the  flower  stalk. 

365.  The  life  history  of  an  angiosperm.    The  formula  for 
the  life  history  of  an  angiosperm  is  the  same  as  that  of  a  gym- 
nosperm  (Sec.  356).    The  gametophyte  phases,  however,  occupy 
generally  a  much  shorter  period,  so  that  the  seeds  are  matured 
in  the  same  season  and   sometimes  within  a  few  weeks  after 
pollination. 

The  formula  is  then  as  follows : 

pollen  grain  —  Male  Gametophyte  —  sperm  nucleus  -. 

SpOTO-  /      (.microspore)  (proto,>la*nn^contents  \ 

Phyte  \  embryosac  _  tfemaie  Gametophyte  -  egg  etc> 

(megaxpore)  {protoplasmic  contents 

of  embryosac) 


THE  ORIGIN  OF  SEED  PLANTS  889 

THE  OKIGIN   OF   SEED  PLANTS  AND   THE 
SEED  HABIT* 

366.  The  origin  of  seed  plants.    We  shall  never  know  exactly 
when  and  how  seed  plants  arose,  for  that  important  event  in 
plant  evolution  probably  took  place  earlier  than  the  Carbon- 
iferous Age.    We  can,  however,  form  some  idea  of  the  chief 
factors  that  brought  about  the   seed  habit  from  a  study   of 
the  life  histories  of  living  pteridophytes  and  spermatophytes. 
As  with  a  number  of  other  forward  steps  in  the  evolution  of 
plants,  such  as  the  origin  of   sex,  alternation  of  generations, 
and  heterospory,  the  seed  habit  probably  was  developed  by  a 
number  of  different  groups  of  pteridophytes  independently  one 
of  another.    Thus  the  cycads  and  the  conifers  among  the  gym- 
nosperms  are  so  widely  separated  that  it  seems  possible  that 
they  may  have   come  from  different  pteridophyte  parentage. 
Therefore  the  gymnosperms  are  generally  regarded  as  a  group  of 
divergent  evolutionary  lines.    The  angiosperms  are  even  more 
puzzling.    Some  botanists  believe  that  they  arose  quite  inde- 
pendently of  the  gymnosperms,  but  others  hold  that  they  may 
be  distantly  related  to  Gnetum.    Some  think  that  the  mono- 
cotyledons and  dicotyledons  even  have  had  independent  origins. 
However,  the  view  which  seems  to  be  finding  greatest  favor  at 
present  regards  the  monocotyledons  not  as  the  ancestors  of  the 
dicotyledons,  but  as  derived  from  primitive  dicotyledons.1 

367.  The  origin  of  the  seed  habit.    The  most  important  fac- 
tors leading  to  the  seed  habit  appear  to  have  been  (1)  heter- 
ospory, (2)  the  retention  of  the  megaspore  in  the  megasporangium 
to  become  the  embryo  sac  in  which  the  female  gametophyte 
develops  parasitically,  and  (3)  the  development  of  the  pollen  tube 
and  its  parasitic  habit  of  growth  through  the  tissues  of  the 

*  To  THE  INSTRUCTOR  :  This  subject  is  very  difficult  and  may  be  omitted. 

1  These  topics  are  far  too  technical  for  consideration  here.  For  reviews  of 
the  various  theories  with  their  evidence,  the  reader  is  referred  to  Coulter 
and  Chamberlain,  Morphology  of  Spermatophytes  (Gymnosperms),  1901  and 
Morphology  of  Angiosperms^  1903. 


390  THE  SPERMATOPHYTES 

sporophyte  to  reach  the  embryo  sac.  These  principles  are  asso- 
ciated with  the  last  stages  in  the  long  processes  of  the  evolution 
of  the  sporophyte  and  the  degeneration  of  the  gametophyte, 
which  is  briefly  outlined  in  the  next  chapter. 

Heterospory  (Sec.  314,  Chapter  xxvn)  has  differentiated  the 
spores  of  the  pteridophytes  and  established  male  and  female 
gametophytes,  the  first  always  developing  from  the  microspores, 
and  the  second  from  the  megaspores.  At  the  same  time  the 
gametophytes  became  largely  or  wholly  dependent  upon  food 
material  stored  in  the  spores,  and  smaller  and  simpler  in  their 
organization,  until  they  degenerated  into  structures  somewhat 
similar  to  those  now  illustrated  in  the  heterosporous  pterido- 
phytes (Marsilia,  Selaginella,  Isoetes,  etc.).  Some  of  them 
finally  lost  all  their  chlorophyll,  and  adopted  parasitic  habits. 

Heterospory  also  resulted  in  the  differentiation  of  the  spore 
leaves  into  microsporophylls  and  megasporophylls,  and  at  last, 
as  in  many  seed  plants,  the  sporophytes  themselves  became 
differentiated,  some  producing  only  pollen  (microspores)  and 
some  only  embryo  sacs  (megaspores)  in  the  ovules.  In  this  way 
sexual  characters  of  the  gametophytes  were  gradually  taken 
up  first  by  the  sporophylls  and  later  by  the  sporophytes  them- 
selves, and  thus  the  asexual  generation  began  to  assume  the 
peculiarities  of  sex.  The  microsporophyll  of  the  seed  plant 
(stamen)  took  on  characteristics  of  a  male  organ,  and  the  mega- 
sporophyll  (carpel)  characteristics  of  a  female  one.  The  early 
botanists  regarded  the  pollen  grain  as  a  male  element  and  the 
stamen  as  a  male  organ,  and  it  is  true  that  these  structures  have 
male  characters ;  but  of  course  the  actual  male  gametes  are  the 
sperm  nuclei  with  closely  associated  protoplasm  in  the  pollen 
tube  whose  contents  represent  a  male  gametophyte.  And  simi- 
larly, although  the  carpel  has  female  characters,  the  female 
gamete  is  the  egg  within  the  embryo  sac  whose  contents  rep- 
resent a  female  gametophyte.1 

1  This  subject  is  considered  more  at  length  in  Chapter  xxvn,  Heter- 
ospory, Sec.  339. 


THE  ORIGIN  OF  THE  SEED  HABIT  391 

The  retention  of  the  megaspore  in  the  megasporangium  was, 
perhaps*  the  most  important  step  in  the  development  of  the  seed 
habit.  This  retention  was  possibly  at  first  somewhat  accidental; 
that  is,  the  megaspore  simply  failed  to  fall  out  of  the  megaspo- 
rangium (as  actually  happens  in.  some  species  of  Selaginella, 
Sec.  325),  and  consequently  developed  its  gametophytes  while 
mechanically  held  on  the  sporophyte.  Later  the  retention  be- 
came more  intimate  and  less  mechanical,  so  that  the  female 
gametophyte  established  a  close  physiological  association  with 
the  sporophyte,  obtaining  protection  and  certain  foods,  and  per- 
haps most  important  of  all  it  was  kept  moist.  At  last  the  mega- 
spore, instead  of  being  developed  as  a  free  cell,  remained  a  part 
of  the  tissue  of  the  megasporangium  (nucellus  of  the  ovule)  and 
at  that  stage  became  the  embryo  sac  with  its  clearly  estab- 
lished parasitic  relations  to  the  sporophyte. 

Some  forms  of  Selaginella  actually  illustrate  a  beginning  of 
such  parasitic  relations  in  the  early  stages  of  the  development 
of  its  megaspore  (Sec.  325),  for  the  female  gametophyte  begins 
to  develop  before  the  spores  are  full  grown  and  ready  to  be 
discharged.  But  the  seed  habit  could  not  have  been  entirely 
formed  until  the  megaspore  became  physiologically  a  part  of 
the  megasporangium,  and  the  latter  (as  a  nucellus),  together 
with  the  protective  integuments,  became  the  ovule. 

The  development  of  the  pollen  tube  is  perhaps  even  more 
remarkable  than  the  retention  of  the  megaspore  in  the  mega- 
sporangium. It  seems  clear  that  the  pollen  tube  is  a  develop- 
ment in  response  to  the  stimulus  of  the  moisture  (containing 
food  substances)  which  is  excreted  by  the  ovule  in  the  gymno- 
sperms  and  the  stigma  of  the  angiosperms.  The  habit  may 
readily  have  had  a  very  simple  beginning  if  microspores  fell 
into  partially  opened  megasporangia,  as  indeed  occurs  in  a  spe- 
cies of  Selaginella  (Sec.  325),  or  among  a  group  of  megasporo- 
phylls.  They  would  have  found  in  such  situations  moisture 
and  other  conditions  favorable  for  the  development  of  out- 
growths which  later  became  tubes.  These  outgrowths  and 


392  THE  SPERMATOPHYTES 

tubes  would  be  expected  to  become  more  and  more  specialized 
as  conditions  arose  which  led  to  the  final  retention  of  the  mega- 
spore  within  the  megasporangium,  and  at  last  they  assumed  pro- 
nounced parasitic  habits. 

With  the  parasitic  habits  of  the  pollen  tube  established  it  is 
not  difficult  to  imagine  the  gradual  adjustment  of  the  peculiar- 
ities of  pollination  to  those  of  ovule  formation.  It  seems  prob- 
able that  the  earliest  forms  of  pollen  tubes  carried  motile  sperms 
to  the  embryo  sac 1  (for  motile  sperms  are  even  now  present  in 
the  cycads  and  Ginkgo,  Sec.  348),  but  later  the  complex  struc- 
ture of  the  sperm  degenerated,  with  that  of  the  whole  male 
gametophyte,  until  the  sperm  nuclei  became  practically  all  that 
was  left  to  represent  the  male  gametes  of  the  pteridophytes, 
bryophytes,  and  algse.  The  simplification  of  the  sperm  and 
egg  in  the  spermatophytes  does  not,  however,  affect  the  signifi- 
cance of  these  sexual  elements,  because  it  is  known  that  the 
nuclei  are  the  most  essential  structures  of  gametes. 

Thus  the  peculiarities  of  the  ovule  and  the  pollen  tube  prob- 
ably developed  side  by  side,  adjusting  themselves  to  one  another 
until  the  complex  phenomena  of  pollination  became  established. 
These  processes  are  relatively  simple  in  the  gymnosperms,  where 
the  pollen  is  applied  directly  to  the  ovule ;  but  in  the  angiosperms 
a  new  feature  was  introduced  when  carpels,  or  groups  of  carpels, 
frequently  with  adjacent  tissue  of  the  stem,  developed  the  ovule 
cases  (ovaries).  Yet  it  is  not  very  difficult  to  understand  how 
they  may  have  arisen,  for  the  same  principles  of  protecting 
the  megaspore  (embryo  sac)  and  providing  for  the  germination 

1  In  certain  fossil  groups  (Pteridospermce),  intermediate  between  the 
pteridophytes  and  spermatophytes,  the  evidence  indicates  that  motile  sperms 
were  discharged  into  large  pollen  chambers  filled  with  water,  into  which 
the  necks  of  the  archegonia  opened  so  that  the  sperms  were  able  to  swim 
directly  to  the  eggs.  The  pollen  tube  was  probably  at  first  an  absorbing 
organ,  or  haustorium  (as  in  the  cycads  and  Gink^o  to-day,  Sec.  348),  pene- 
trating the  tissue  of  the  ovule  to  obtain  nourishment  for  the  parasitic  male 
gametophyte.  Later,  with  the  disappearance  of  the  pollen  chamber  and 
motile  sperms,  the  pollen  tube  took  on  the  added  function  of  carrying  the 
sperm  nuclei  directly  to  the  embryo  sac. 


THE  EVOLUTION  OF  THE  FLOWER  393 

of  the  pollen  grains  are  simply  carried  one  step  farther,  and 
the  megasporophylls  (carpels)  become  factors  in  the  processes. 
Thus  a  receptive  surface,  the  stigma,  was  developed  as  a  special 
organ  to  receive  and  start  the  pollen  tube  in  its  parasitic  devel- 
opment, which  is  to  end  with  the  fertilization  of  the  egg. 

The  seed  is  the  ripened  ovule,  for  the  principle  of  protection 
is  continued  after  fertilization,  and  the  integuments  form  hard 
seed  coats,  inclosing  the  developing  embryo,  supplied  with  food 
material  by  the  parent  sporophyte  until  it  has  reached  an  ad- 
vanced stage  of  development. 

THE  EVOLUTION  OF  THE  FLOWER 

368.  The  evolution  of  the  flower.*  The  higher  types  of 
flowers  have  been  developed  by  long  processes  of  evolution  from 
the  simpler  structure  of  the  primitive  flowers.  We  do  not 
know  exactly  what  the  primitive  flowers  were  like,  but  some 
of  their  characters  may  be  inferred  from  the  structure  of  the 
simplest  flowers  of  the  angiosperms  and  the  cones  of  gymno- 
sperms  and  certain  pteridophytes,  as  the  horsetails  and  club 
mosses,  which  are  truly  flowers,  if  one  accepts  the  definition  of 
a  flower  as  a  "  shoot  beset  with  sporophylls."  The  most  elabo- 
rately developed  theory  of  floral  evolution  is  that  of  Engler,  and 
this  brief  outline  will  be  a  general  statement  of  his  views. 

Primitive  flowers  were  characterized  by  indefinite  numbers 
of  sporophylls,  usually  distributed  in  spirals,  and  the  absence  of 
the  floral  envelopes  constituting  a  perianth.  These  conditions 
are  illustrated  in  the  cones  of  the  pteridophytes  and  in  many 

*  To  THE  INSTRUCTOR  :  This  subject  should  only  be  presented  to  classes 
with  a  fairly  wide  range  of  experience  with  flower  structure  in  various 
groups  of  angiospern.s.  An  excellent  study  would  be  a  series  of  types  from 
such  an  assemblage  as  the  buttercup  order,  Ranunculales,  as  the  mouse- 
tail  (Myosurus),  buttercups,  magnolia,  white  water  lilies,  columbine,  lark- 
spur, aconite,  etc.,  where  many  of  the  principles  of  flower  evolution  are 
illustrated  in  a  single  group.  Similar  studies  might  be  planned  for  the  rose 
order,  Rosales,  or  the  lily  order,  Liliales,  followed  by  the  orchids. 


394  THE   SPERMATOPHYTES 

gymnosperms.  Spiral  arrangements  of  sporophylls  (stamens  and 
carpels)  and  floral  envelopes  are  also  not  uncommon  in  many 
flowers  with  well-developed  perianths,  as  in  representatives  of  the 
buttercup  order,  fianunculales,  namely,  mousetail  (Myosurus),  but- 
tercups, magnolia,  white  water  lilies,  and  the  rose  order,  Mosaics. 
It  is  not  at  all  probable  that  the  various  advances  over  the 
primitive  conditions  followed  any  regular  order.  Some  of  them 
were  concerned  with  the  differentiation  of  a  perianth ;  some  had 
to  do  with  the  arrangements  of  the  sporophylls  and  parts  of  the 
perianth;  some  dealt  with  the  apparent  fusion  of  parts,  and 
some  concerned  the  symmetry  of  the  flower. 

The  differentiation  of  a  perianth  has  clearly  taken  place  in 
some  flowers  through  the  transformation  of  sporophylls,  which 
became  sterile  and  assumed  perianth  characters  (generally  those 
of  petals).  Such  transformations  are  admirably  shown  in  the 
passage  of  stamens  into  the  parts  of  the  perianth  in  the  white 
water  lily,  and  in  the  doubling  of  flowers,  where  stamens  and 
frequently  carpels  become  petals.  It  is  possible,  however,  that 
parts  of  a  perianth  may  be  derived  in  a  reverse  direction, —  that 
is,  from  leaves  or  bracts  on  the  stem  just  below  the  sporophylls. 
That  ordinary,  leaves  can  become  highly  modified  and  colored 
to  serve  the  purpose  of  a  perianth  is  illustrated  by  the  showy 
bracts  of  the  painted  cup,  or  the  flowering  dogwood  and  other 
species  of  Cornus  (Frontispiece).  The  parts  of  the  simplest  types 
of  perianth  were  probably  all  similar  and  largely  protective, 
especially  to  the  flower  bud.  These  later  became  differentiated 
into  the  two  sets,  sepals  and  petals,  —  the  latter,  and  frequently 
also  the  former,  showy  and  clearly  related  to  pollination  by 
insects  or  birds  (Chapter  xxxn). 

The  arrangements  of  the  sporophylls  and  parts  of  the  peri- 
anth are,  as  a  rule,  spiral  in  simpler  types  of  flowers,  but  gener- 
ally in  circles  or  whorls  in  higher  types.  In  passing  from  the 
spiral  to  the  cyclic  arrangements  the  variable  and  indefinite 
numbers  of  parts  tend  to  become  constant.  Thus  three  and 
multiples  of  three  are  the  prevailing  numbers  in  the  flowers  of 


THE  EVOLUTION  OF   THE  FLOWER  395 

monocotyledons,,  while  four  and  five  are  common  numbers  in 
the  dicotyledons.  A  settling  of  the  parts  into  fixed  numbers 
would  be  always  an  important  forward  step  in  floral  evolution, 
according  to  Engler,  whether  it  concerns  the  perianth,  the 
sporophylls,  or  both  together;  for  it  tends  to  give  definite  form 
to  the  flower,  and  thus  leads  toward  the  higher  conditions. 
Sometimes  a  flower  will  be  mixed  in  the  arrangement  of  its 
parts,  the  perianth  being  cyclic  and  the  stamens  and  carpels 
spiral,  as  in  certain  buttercups.  The  establishment  of  fixed 
numbers  is  frequently  accompanied  by  the  suppression  of  some 
parts  (sepals,  petals,  stamens,  or  carpels),  so  that  the  numbers 
are  variable  in  different  circles. 

The  apparent  fusion  of  parts,  frequently  called  coalescence, 
results  from  the  formation  of  tubular  or  cup-like  outgrowths 
from  zones  of  tissue  below  the  floral  parts,  so  that  they  seem  to 
be  united.  The  most  complex  conditions  of  flower  structure, 
called  epigyny  (Fig.  136,  C)  and  perigyny  (Fig.  136,  B),  are  due 
to  these  zonal  growths  (see  Sees.  152,  157,  364).  The  contrast 
to  epigyny  and  perigyny  is  hypogyny  (Fig.  136,  A).  When 
petals  or  sepals  are  borne  on  zonal  outgrowths  the  conditions 
are  called,  respectively,  sympetaly  and  synsepaly  (Sec.  152).  The 
compound  pistil,  —  that  is,  a  pistil  involving  two  or  more  car- 
pels, —  is  one  of  the  highest  expressions  of  zonal  growth  and  is 
called  syncarpy  (meaning  united  fruits). 

The  symmetry  of  the  flower  may  be  either  radial  or  bilateral, 
that  is  with  a  right  and  a  left  half  (Sec.  150).  Primitive  flowers 
were  radially  symmetrical,  as  would  be  expected  from  an  in- 
definite number  of  parts  spirally  arranged.  Bilateral  symmetry 
appears,  however,  in  very  many  groups  and  always  represents  a 
high  degree  of  floral  evolution.  It  is  found  more  commonly  in 
epigynous  and  perigynous  flowers  than  in  hypogynous,  but  there 
is  no  rule  about  its  relations  to  these  conditions.  Bilateral  sym- 
metry is  usually  directly  related  to  methods  of  flower  pollination 
by  insects,  for  the  forms  of  such  flowers  are  especially  adapted  to 
the  habits  of  bees,  which  light  on  some  expanded  lip-like  region 


396  THE   SPERMATOPHYTES 

of  the  perianth  and  rummage  around,  gathering  pollen  and 
nectar,  and  incidentally  effecting  the  pollination  of  the  stigma 
(Sec.  401). 

Bilateral  symmetry  is  generally  accompanied  by  dorsiven- 
trality,  which  means  that  the  flower  hangs  in  such  a  position 
that  there  is  an  upper  and  a  lower  portion  as  well  as  a  right 
and  a  left  half.  Excellent  illustrations  are  such  lipped  flowers  as 
the  snapdragons,  the  mints,  and  many  orchids.  An  epigynous 
flower  whose  symmetry  is  bilateral  and  dorsiventral  and  whose 
parts,  through  suppression  or  other  developments,  show  irregu- 
larities which  have  a  clear  relation  to  insect  visitations, — these 
characters  give  the  highest  types  of  flower  evolution. 

According  to  Engler,  the  chief  steps  in  the  evolution  of  the 
flower  may  be : 

1.  The  differentiation  of  a  perianth. 

2.  The  change  from  spiral  arrangement  of  parts,  with  indefi- 
nite numbers,  to  cyclic  arrangements,  with  fixed  numbers. 

3.  The  grouping  of  parts  through  zonal  growths  (coalescence), 
resulting  in  syncarpy,  perigyny,  and  epigyny. 

4.  The  change  from  radial  to  bilateral  symmetry,  accompa- 
nied by  dorsiventrality. 

5.  To  these  stages  in  floral  evolution  should  be  added  the 
complexity  attained  by  the  massing  of  numerous  flowers  in 
groups  or  heads  (Sec.  165),  as  in  the  composite  family  (daisies, 
sunflowers,  etc.).    In  the  highest  expressions  of  this  development 
the  flowers  are  differentiated  so  that  the  outermost  of  the  groups 
become  sterile,  but  by  a  remarkable  lengthening  of  their  corollas 
into  rays  the  flower  cluster  becomes  very  conspicuous. 

All  flowers  do  not,  by  any  means,  follow  the  order  of  evolution 
as  outlined  above,  and  there  are  very  many  special  irregularities 
in  different  groups.  Thus  certain  flowers  of  the  legume  family 
are  bilaterally  symmetrical  and  dorsiventral,  but  there  is  no 
perigyny  or  epigyny.  It  is  important  to  note  that  the  higher 
levels  of  flower  evolution  have  been  developed  again  and  again  in 
unrelated  groups  of  angiosperms  independently  one  of  another 


THE  MONOCOTYLEDONS  397 

(as,  for  example,  among  the  orchids,  the  legumes,  the  snap- 
dragons, the  mints,  etc.).  While  there  is  generally  an  upward 
evolution  of  flowers,  especially  when  insect-pollinated,  there  are 
in  some  groups  numerous  illustrations  of  floral  degeneration. 

THE  CLASSIFICATION  OF  THE  ANGIOSPEEMS 

369.  The  classification  of  the  angiosperms.1   The  subdivision 
Angiospermce  contains  two  classes  : 

CLASS    I.  The  monocotyledons,  or  Monocotyledonece ,  with  an  embryo 

having  a  single  lateral  cotyledon. 
CLASS  II.  The  dicotyledons,  or  Dicotyledonece,  with  an  embryo  having 

two  terminal  cotyledons  (including  a  few  exceptions). 
SUB-CLASS  1.  The  Arcliicldamydece  (meaning  primitive  floral  envel- 
opes), in  which  the  perianth  is  wanting,  or,  if  pres- 
ent, has  its  parts  entirely  separate  from  one  another. 
SUB-CLASS  2.  The  Metacldamydece  (meaning  later  floral  envelopes),  or 
Sympetalce,  in  which  the  petals  are  united  or  borne 
on  tubular,  ^cup-like,  or  other  forms  of  zonal  out- 
growths from  the  receptacle  (Sec.  152). 

370.  The  monocotyledons.    Besides  having  the  single  coty- 
ledon in  the  embryo,  this  group  is  distinguished  from  the  dicoty- 
ledons by  having  a  stem  structure,  with  scattered  fibro-vascular 
bundles,  instead  of  a  cyclic  arrangement.    Consequently  there 
can  be  no  development  of  a  central  shaft  of  wood  surrounded 
by  a  cylinder  of  bast,  with  a  cambium  tissue  lying  between  the 
two,  as  is  commonly  found  in  the  larger  dicotyledons.  The  leaves 
are  generally  closed  (parallel)  veined  instead  of  open  (netted) 
veined,  and  rarely  notched,  which  means  that  their  fibro-vascular 
bundles  come  together  at  the  tip  or  along  the  edge  of  the  leaf, 
instead  of  ending  freely  as  they  do  in  the  dicotyledons.    The 
parts  of  the  flower  are  generally  in  three  or  multiples  of  three. 

1  The  most  generally  accepted  classification  of  the  angiosperms  is  that  of 
Engler,  presented  in  the  Syllabus  der  Pflanzenfamilien,  1903.  A  brief  state- 
ment of  the  chief  features  of  this  system  will  be  found  in  Coulter  and  Cham- 
berlain, Morphology  of  Angiosperms,  1903. 


398  THE  SPERMATOPHYTES 

There  are  more  than  20,000  species  of  monocotyledons,  which 
are  arranged  by  Engler  into  11  orders,  the  chief  of  which  are : 

1.  The  grass  and  sedge  order,  Graminales,  including  more 
than  6000  species,  one  of  the  most  successful  assemblages  of 
angiosperms  and  by  far  the  largest  in  the  number  of  individuals. 

2.  The  palm  order,  Palmales,  a  very  characteristic  tropical 
and  sub-tropical  group. 

3.  The  lily  order,  Liliales,  a  large  group  of  almost  5000 
species,  remarkable  for  the   showiness   and  symmetry  of  its 
flowers. 

4.  The  orchid  order,  Orchidales,  containing  the  large  orchid 
family  with  more  than  5000  species,  the  largest  family  in  the 
Monocotyledonece,  and  one  of  the  most  remarkable  groups  of  seed 
plants  for  the  beauty  and  complexity  of  its  flowers  and  for  its 
peculiar  life  habits. 

371.  The  dicotyledons.   Besides  having  two  cotyledons  in  the 
embryo,  this  group  is  distinguished  from  the  monocotyledons 
by  having  its  fibro-vascular  bundles  formed  in  a  circle.    This 
arrangement  makes  possible  the  development  of  a  central  shaft 
of  wood  (xylem),  since  the  cambium  regions  of  the  bundles 
unite  into  a  cylinder  which  adds  successive  layers  of  wood  if 
the  plant  is  perennial.    The  bundles  in  the  leaves  are  strongly 
developed,  much  branched,  and  end  freely,  so  that  the  leaves 
are  conspicuously  open  (netted)  veined,  generally  notched,  and 
frequently  deeply  divided,  or  compound.    The  parts  of  the  flowers 
are  mostly  in  fours  and  fives  in  the  higher  types,  except  that  the 
number  of  carpels  is  commonly  less. 

There  are  more  than  100,000  species  of  dicotyledons,  and 
these  are  arranged  by  Engler  into  34  orders  (26  in  the  Archi- 
chlamydece,  and  8  in  the  Metaclilamydece). 

372.  The  Archichlamydeae.    This  sub-class  is  an  immense 
assemblage,  very  diverse  in  character,  whose  flowers  range  from 
primitive  types,  with  indefinite  numbers  of  parts  in  spiral  arrange- 
ments, to  cyclic  flowers  with  definite  numbers,  perigyny,  epig- 
yny,  and  syncarpy.    Some  of  the  chief  orders  are : 


THE  ARCHICHLAMYDE/E  399 

1.  The  tree  orders,  including  the  willows  and  poplars  (Sali- 
cales);  the  walnuts  and  hickories  (Juglandales) ;  the  birches, 
alders,  beech,  chestnut,  and  oaks  (Fag ales) ;  the  elms,  figs,  mul- 
berries, etc.  (Urticales). 

2.  The  buttercup  order,  Ranunculales,  a  large  assemblage  of 
about  4000  species,  full  of  interesting  gradations  in  floral  evolu- 
tion, the  buttercup  family  (Ranunculacece)  being  an  especially 
good  group  for  such  studies. 

3.  The  poppy  order,  Papaverales,  comprising  the  poppies  and 
the  large  mustard  family. 

4.  The  rose  order,  Rosales,  an  immense  group  of  over  14,000 
species,  with  several  large  families,  such  as  the  legume  or  pea 
family,  the  rose  family,  etc.    The  flowers  present  a  greater  range 
of  structure  than  in  the  buttercup  order.    Some  large  groups  in 
the  legume  family  have  flowers  with  well-developed  bilateral 
symmetry  and  dorsiventrality. 

5.  The  geranium  order,  Geraniales,  containing  the  geraniums, 
flax,  Euphorbias,  etc. 

6.  The  violet  order,  Violales,  comprising  a  large  number  of 
families  and  more  than  4000  species. 

7.  The  cactus  order,  Cactales,  a  very  remarkable  American 
group  of  more   than   900    species,  mostly  adapted  to  desert 
conditions. 

8.  The  umbel  order,  Umbellales,  containing  more  than  2500 
species,  mostly  in  the  umbel  (parsley)  and  dogwood  families,  — 
the  highest  order  in  the  series  of  the  ArchiMamydece  on  account 
of  its  epigynous  flowers,  the  reduced  number  of  carpels,  and  the 
massing  of  the  flowers  in  the  characteristic  umbel,  or  in  close 
heads  surrounded  by  a  corolla-like  involucre  of  bracts,  as  in  the 
dogwoods  (Cornacece,  see  Frontispiece). 

373.  The  Metachlamydeae.  The  general  flower  characters  of 
this  sub-class  are  cyclic  arrangements  of  parts  with  definite 
numbers,  perigyny  or  epigyny,  and  a  reduced  number  of  carpels 
in  the  compound  pistil  (syncarpy).  The  corollas  are  usually 
showy,  the  petals  being  borne  on  tubular  or  cup-like  outgrowths 


400  THE  SPERMATOPHYTES 

(sympetaly).  The  stamens  are  usually  also  borne  on  the  same 
outgrowth  with  the  petals,  so  that  they  appear  to  arise  from 
them  (epipetaly).  The  chief  orders  are: 

1.  The  ericad  order,  Ericales,  containing  the  heath  family,  a 
very  characteristic  group  in  the  northerly  parts  of  America, 
Europe,  and  Asia,  especially  in  the  mountains. 

2.  The  gentian  order,    Gentianales,  with    more  than  4000 
species,  including  the  gentians,  olive  family,  milkweeds,  etc. 

3.  The  phlox  order,  Polemoniales,  with  more  than  14,500  spe- 
cies, containing  a  number  of  prominent  families,  as  the  phloxes, 
borrages,  nightshades,  figworts,  mints,  verbenas,  and  others.    The 
two-lipped  flowers  of  the  mints,  figworts,  etc.,  distinguish  these 
families  among  the  MetacJilamydece  as  the  legumes  are  distin- 
guished among  the  ArchiMamydece,  and  the  orchids  among  the 
monocotyledons. 

4.  The  madder  order,  Rubiales,  including  the  large  madder 
family,  the  honeysuckles,  the  valerian  family,  and  the  teasels. 

5.  The  bellwort  order,  Campanulales,  containing  the  highest 
of  all  angiosperm  families,  the  Composite,  the  largest  in  the 
number  of  species  (more  than  12,000),  and  one  of  the  most 
successful  groups  of  plants. 

SUMMARY  OF    THE   SPEKMATOPHYTES   AND   THEIE 
ADVANCES  OVER  THE  PTEKIDOPHYTES 

374.  Summary  of  the  spermatophytes.  The  chief  charac- 
ters of  the  spermatophytes  and  their  advances  over  the  pteri- 
dophytes  are : 

1.  The  retention  of  the  inegaspore  as  an  intimate  part  of  the 
megasporangium  (nucellus)  to  become  the  embryo  sac,  and  the 
development  of  the  female  gametophyte  parasitically  within  this 
structure.    The  degeneration  of  the  female  gametophyte  in  the 
angiosperms  to  a  group  of  nuclei  within  the  embryo  sac. 

2.  The  origin  of  the  ovule  as  a  new  structure  from  the  mega- 
sporangium  (nucellus),  together  with  enveloping  integuments. 


SUMMARY   OF  THE  SPERMATOPHYTES  401 

3.  The  development  of  the  pollen  tube  from  the  microspore 
(pollen  grain)  as  a  result  of  the  habit  of  pollination,  by  which 
microspores  enter  the  micropyles  of  the  ovules  in  gymnosperms, 
and  fall  upon  a  receptive  structure,  the  stigma,  in  angiosperms. 

4.  The  degeneration  .of  the  male  gametophyte   until  it  is 
hardly  more  than  a  group  of  nuclei,  with  accompanying  pro- 
toplasm, in  the  pollen  grain  and  its  tube.    The  degeneration  of 
the  motile  sperms  until  they  are  represented  by  two  sperm 
nuclei  alone  (cycads  and  Ginkgo  excepted,  Sec.  348),  which  are 
carried  by  the  pollen  tube  into  the  embryo  sac. 

5.  The  development  and  retention  of  the  embryo  sporophyte 
within  the  embryo  sac,  and  the  ripening  of  the  ovule  into 
the  seed. 

6.  The  massing  of  the  sporophylls  on  the  shoot,  accompanied 
by  envelopes  which  constitute  the  perianth  of  the  flower.    The 
development  in  the  angiosperms  of  the  megasporophyll,  or  carpel, 
into  the  simple  pistil,  and  the  grouping  of  carpels  through  zonal 
growth  (syncarpy)  to  form  the   compound  pistil,  so  that  the 
ovules  become  inclosed  in  an  ovule  case  (ovary).    The  differ- 
entiation of  a  receptive  surface,  the  stigma,  on  the  pistil  upon 
which  the  pollen  grain  may  germinate. 

7.  The  differentiation  of  the  parts  of  the  perianth  into  sepals 
and  petals,  and  their  grouping  through  zonal  growth,  together 
with  the  stamens,  to  give  perigyny,  epigyny,  sympetaly,  syn- 
sepaly,  and  epipetaly.    The  development  of  bilateral  symmetry 
and  dorsiventrality. 

8.  A  general  development  of  the  sporophytes  in  many  par- 
ticulars, giving  them  much  greater  complexities  of  tissue  struc- 
ture, growth,  and  form  than  those  of  the  pteridophytes. 


CHAPTEE  XXIX 

THE  EVOLUTION  OF  THE  SPOROPHYTE  AND  DEGENERATION 
OF  THE  GAMETOPHYTE 

375.  The  evolution  of  the  sporophyte.  Alternation  of  genera- 
tions had  its  beginnings  among  the  thallophytes,  and  is  clearly 
shown  in  the  life  histories  of  the  red  algae  and  the  sac  fungi, 
but  is  not  so  conspicuous  there  as  in  the  higher  divisions  of 
the  plant  kingdom.  Furthermore,  the  sporophyte  generations  of 
these  thallophytes  do  not  seem  to  be  related  to  the  sporophytes 
of  the  liverworts  and  the  groups  above  them,  but  are  probably 
of  independent  origin. 

Consequently  the  line  of  evolution,  with  the  remarkable 
development  of  the  sporophyte  and  degeneration  of  the  game- 
tophyte,  as  illustrated  by  the  pteridophytes  and  spermatophytes, 
really  had  its  beginning  in  the  lower  bryophytes  and  in  the  algal 
ancestry,  probably  Chloropliycece,  from  which  they  were  derived. 
This  algal  ancestry,  however,  is  not  known,  for  there  are  no 
living  algse  that  have  the  combination  of  characters  which  would 
be  expected  of  the  ancestors  of  the  bryophytes,  —  namely,  the 
multicellular  sexual  organs,  together  with  clearly  established 
sporophyte  and  gametophyte  phases  in  the  life  histories. 

The  bryophytes  were  responsible  for  the  first  great  steps  in 
the  evolution  of  the  sporophyte  toward  the  conditions  presented 
in  the  ferns  and  seed  plants.  All  of  the  sporophytes  of  the 
liverworts  and  mosses  are  to  a  great  extent  parasitic  upon  the 
gametophytes;  that  is,  they  take  water  from  them,  and  probably 
certain  foods  in  solution.  Two  important  advances  appeared 
in  the  bryophytes. 

First.  The  spore-forming  tissue  gradually  came  to  occupy  a 
relatively  smaller  part  of  the  sporophyte  (compare  the  sporophytes 

402 


THE  EVOLUTION  OF  THE   SPOROPHYTE  403 

of  the  Riccia  group  with  those  of  Marchantia,  Porella,  and 
Anthoceros).  Thus  tissue  which  originally  developed  spores, 
or  had  spore-forming  possibilities,  became  set  apart  for  vegeta- 
tive functions  alone.  In  this  manner  the  foot  and  stalk  became 
established  in  Marcliantia  and  Porella,  and  the  heavy  walls  of 
the  spore  case  in  Anthoceros.  This  principle  has  been  called  the 
"  sterilization  of  potential  sporogenous  tissue,"  but  a  simpler  ex- 
pression would  be  "  the  assumption  of  vegetative  functions  by 
tissues  with  spore-forming  possibilities." 

Second.  Portions  of  the  chlorophyll-bearing  regions  of  the 
sporophytes  developed  stomata  in  Anthoceros  and  in  some  of 
the  mosses,  and  this  was  the  beginning  of  the  elaborate  mechan- 
ism for  chlorophyll  work  (photosynthesis),  which  is  developed  to 
such  a  high  degree  in  the  leaves  of  ferns  and  seed  plants. 

The  pteridophytes  carried  the  advance  much  farther,  through 
the  third,  fourth,  fifth,  and  sixth  great  steps  in  the  development 
of  the  sporophyte. 

Third.  The  sporophyte  became  independent  of  the  gameto- 
phyte  by  developing  roots,  and  to  these  added  stems  and  fronds. 

Fourth.  This  condition  was  associated  with  the  differentia- 
tion of  a  vascular  tissue  that  made  it  possible  for  the  sporophyte 
to  grow  to  a  considerable  height  above  the  ground,  (1)  by  ena- 
bling it  to  maintain  a  connection  with  a  water  supply  through  the 
roots,  and  (2)  by  providing  it  with  a  strong  framework  through- 
out the  stem  and  leaves.  In  their  strengthening  functions  the 
nbro-vascular  bundles  were  greatly  assisted  by  the  development 
of  rigid  tissues  (schlerenchyma).  In  other  respects,  also,  the 
entire  tissue  structure,  or  histology,  of  the  sporophyte  became 
much  more  complicated. 

Fifth.  Fronds  were  differentiated  into  spore  leaves,  or  spo- 
rophylls,  and  vegetative,  or  foliage  leaves.  The  spore  leaves 
became  grouped  into  cones,  and  by  heterospory  were  differen- 
tiated into  microsporophylls  and  megasporophylls. 

Sixth.  The  embryo  sporophyte  of  heterosporous  pteridophytes, 
through  the  shortening  of  the  gametophytic  phases,  came  to  use 


404  THE  EVOLUTION  OF  THE  SPOROPHYTE 

and  depend  upon  food  stored  in  the  megaspores  by  the  previous 
sporophyte  generation. 

The  spermatophytes  added  the  final  stages  in  the  evolution 
of  the  sporophyte,  as  follows : 

Seventh.  The  ovule  arose  through  the  retention  of  the  mega- 
spore  (embryo  sac)  in  the  megasporangium  (nucellus)  inclosed 
by  integuments.  The  development  of  the  embryo  sporophyte 
within  the  embryo  sac,  and  the  ripening  of  the  ovule,  produced 
the  seed. 

Eighth.  The  ovule  case,  or  ovary,  appeared  with  the  devel- 
opment from  one  or  more  megasporophylls,  or  carpels  (frequently 
with  adjacent  tissue),  of  an  inclosing  structure,  the  pistil,  upon 
which  was  differentiated  a  special  region,  the  stigma,  for  the 
reception  of  the  pollen. 

Ninth.  The  stamen  was  developed  from  the  microsporo- 
phyll. 

Tenth.  Complicated  flowers  arose  by  various  groupings  of  the 
carpels  and  stamens,  together  with  showy  or  protective  envelopes 
constituting  the  perianth. 

Eleventh.  The  flower  cluster,  or  inflorescence,  appeared,  culmi- 
nating in  the  composite  head. 

Twelfth.  The  tissues  of  the  spermatophytes  became  more  com- 
plicated in  many  respects  than  those  of  the  pteridophytes. 

376.  The  degeneration  of  the  gametophyte.  Many  steps  in 
the  degeneration  of  the  gametophyte  were  closely  related  to  the 
advances  of  the  sporophyte. 

In  most  of  the  bryophytes  the  gametophytes  appear  as 
organisms  equally  complex  with  the  sporophytes,  and  in  many 
forms  they  are  more  complex.  Thus  the  gametophytes  of  the- 
mosses  and  leafy  liverworts  show  a  considerable  advance  over 
the  thalloid  gametophytes  of  the  simple  bryophytes.  The  thal- 
loid  gametophyte,  however,  seems  to  have  been  the  type  that 
was  passed  over  to  the  pteridophytes,  and  Anthoceros  probably 
gives  a  fair  idea  of  the  relative  complexity  of  the  two  genera- 
tions at  the  time  when  the  first  pteridophytes  arose. 


DEGENERATION  OF  THE  GAMETOPHYTE    405 

The  beginnings  of  the  degeneration  of  the  gametophyte 
became  clearly  evident  in  the  pteridophytes  when  the  rela- 
tively small  and  simple  prothallium  took  the  place  of  the  large 
gametophytes,  as  illustrated  in  the  Riccia  and  Marchantia 
groups.  Its  further  simplification  was  greatly  accelerated  by 
heterospory,  passing  through  four  prominent  stages : 

First.  Dependence  upon  food  stored  in  the  microspore  and 
rnegaspore,  together  with  gradual  loss  of  chlorophyll,  reduced 
the  gametophytes  to  small  structures  producing  relatively  few 
sexual  organs  and  gametes.  Thus  the  gametophytes  in  the 
pteridophytes  became  dependent  upon  food  supplied  by  the  spo- 
rophytes  by  way  of  the  spores,  —  a  relation  exactly  the  reverse 
of  that  in  the  bryophytes. 

Second.  The  gametophytes  became  differentiated  as  male  and 
female  in  sex,  associated  with  the  microspores  and  megaspores, 
respectively. 

The  spermatophytes,  by  means  of  the  seed  habit,  brought 
about  the  greatest  changes  in  the  gametophytes,  as  follows : 

Third.  The  female  gametophyte  degenerated  to  such  an 
extent  by  the  retention  of  the  megaspore  (embryo  sac)  in  the 
megasporangium  (nucellus)  that  the  archegonium  lost  its  form 
and  finally  became  represented  in  its  essentials  by  the  egg  alone. 
The  vegetative  tissue  became  reduced  until  only  a  few  nuclei 
of  uncertain  relationship  (antipodal  and  polar  nuclei)  remain. 

Fourth.  The  male  gametophyte  degenerated  in  structure  in 
a  similar  manner  until  the  antheridium  disappeared,  and  the 
numerous  ciliated  sperms  of  the  pteridophytes  were  represented 
by  only  two  sperm  nuclei,  with  associated  protoplasm.  Vegeta- 
tive tissue  was  reduced  until  only  a  single  nucleus  remained  in 
the  angiosperms  to  represent  sterile  cells  of  a  male  gametophyte. 

There  arose,  however,  by  means  of  the  seed  habit  an  activity 
on  the  part  of  the  male  gametophyte  which  is  one  of  the  most 
remarkable  developments  in  plant  evolution.  The  appearance  of 
the  pollen  tube,  with  its  parasitic  relations  to  the  sporophy  te,  is 
a  very  complex  life  relation.  This  was  the  chief  cause  of  floral 


406  THE  EVOLUTION  OF  THE  SPOROPHYTE 

evolution,  with  all  its  wonderful  diversities  of  form  and  struc- 
ture in  relation  to  insect  life,  —  diversities  assumed  to  carry  out 
the  relation  of  flowers  to  insect  carriers  of  pollen. 

Part  II  of  this  work  has  given  an  outline  of  the  evolution 
and  classification  of  plants  based  on  comparative  studies  of 
their  morphology.  The  conclusions  are  necessarily  speculative 
and  philosophical,  for  we  have  no  means  of  knowing  exactly 
what  has  happened  throughout  the  geological  ages.  The  fossil 
remains  of  plants  are  very  helpful  in  certain  groups,  as  the  pteri- 
dophytes  and  spermatophytes,  but  they  are  fragmentary  and 
relatively  few,  except  for  such  periods  as  those  when  coal  or 
coal-like  deposits  were  formed.  Consequently  our  conclusions 
as  to  the  evolutionary  history  of  plants  must  be  founded  chiefly 
on  studies  of  life  histories  and  the  comparative  morphology  of 
living  groups.  In  spite  of  difficulties,  the  plant  morphologist  has 
been  able  to  establish  a  classification  of  plants,  based  on  kinship, 
so  as  to  determine  the  framework  of  evolutionary  lines  with 
remarkable  clearness,  and  these  conclusions  give  to  botany  its 
chief  interest  on  the  side  of  morphology.  It  is  not  strange  that 
the  development  of  exact  ideas  in  regard  to  plant  evolution 
should  have  lagged  behind  the  progress  made  in  that  line  in 
animal  evolution,  since  the  paleontological  evidence  available  for 
the  botanist,  as  above  stated,  is  so  scanty.  It  is  only  within  a 
very  few  years  that  any  attempt  has  been  made  to  introduce 
beginners  in  botany  to  the  evolutionary  history  of  plants,  and 
popular  knowledge  of  the  subject  is  now  no  farther  advanced 
than  was  knowledge  of  animal  evolution  more  than  thirty 
years  ago. 


PART  III 

ECOLOGY  AND  ECONOMIC  BOTANY 

CHAPTER   XXX 
PARASITES  AND  CARNIVOROUS  PLANTS 

377.  Ecology.*   Plant   ecology  discusses  the   way  in  which 
plants  get  on  with  their  animal  and  plant  neighbors  and,  above 
all,  the  way  in  which  they  adjust  themselves  to  the  nature  of 
the  soil  and  climate   in   which  they  live.    Ecology,  in  short, 
treats  of  the  relations  of  plants  to  the  world  about  them.    A 
good  deal  of  what  has  been  said  in  previous  chapters  on  such 
topics  as  parasitic  plants,  climbing  plants,  the  movements  of 
leaves,  the  coating  of  hairs  on  stems  and  leaves,  the  storage 
of  water  in  epidermis  cells,  is  really  ecological  botany,  although 
it  is  not  so  designated  in  the  sections  where  it  occurs.    It  is 
evident  enough  that  much  of  the  subject-matter  of  ecology  is 
merely  a  special  department  of  physiology,  but  another  portion 
of  it  forms  an  important  part  of  plant  geography. 

378.  Parasites.    By   the   term  parasite  in  botany,  a  plant 
is   meant   which    draws   its    food   supply   wholly   or  partially 
from  another  living  plant  or  animal  called  the  host.    In  Sec.  29 
the  life  history  of  a  familiar  parasite,  the  dodder,  was  briefly 
sketched,  and  the  parasitic  fungi  among  spore  plants  have  been 
discussed  in  Chapter  xxn. 

*  To  THE  INSTRUCTOR  :  The  treatment  of  the  subject  of  ecology  will  per- 
tain almost  entirely  to  seed  plants.  Many  ecological  topics  relating  to  spore 
plants  have  been  discussed,  under  the  various  groups  described  in  Part  II. 

407 


408  PARASITES  AND  CARNIVOROUS  PLANTS 

379.  Half -parasitic  seed  plants.    Half  parasites,  or  partial 
parasites,  are  those  which  take  a  portion  of  their  food,  or  of 
raw  materials  to  make  food,  from  their  host  and  manufacture 
the  rest  for  themselves.    Usually  they  take  mainly  the  newly 
absorbed  soil  water  from  the  host  and  do  their  own  starch 
making  by  combining  the  carbon  dioxide,  which  they  absorb 
through  their  leaves,  with  the  water  stolen  by  the  parasitic 
roots,  or  Jiaustoria,  imbedded  in  the  wood  of  the  host.    Evidently 
the  needed  water  may  just  as  well  be  taken  from  the  under- 
ground parts  of  the  host  as  from  the  upper  portions,  and  accord- 
ingly many  half  parasites  are  parasitic  on  roots.    This  is  the 
case  with   many   of  the   beautiful  false   foxgloves   (Gerardia), 
with  the  painted  cup   (Castillea),  and   some  species  of  false 
toadflax    (Comandra)  and  some  orchids.1    Usually  these  root 
parasites  are  not  recognized  by  non-botanical  people  as  para- 
sites at  all,  but  in  Germany  a  species  common  in  grain  fields2 
and  the  eyebright,  which  abounds  in  grass  fields,  are  respectively 
known  as  "  hunger  "  and  "  milk  thief,"  from  the  injury  they  do 
to  the  plants  on  which  they  fasten  themselves.    The  mistletoe 
is  a  familiar  example  of  a  half  parasite  which  roots  on  branches. 
Among  the  scanty  belts  of  cotton  wood  trees  along  streams  in 
New  Mexico  it  is  necessary  to  lop  off  the  mistletoe  every  year 
to  give  the  tree  any  chance  to  grow.    Half  parasites  may  be 
known  from  plants  that  are   fully  parasitic   by  having  green 
or  greenish  foliage,  while  complete  parasites  have  no  chlorophyll 
and  so  are  not  at  all  green. 

380.  Wholly  parasitic    seed   plants.    These  are    so   nearly 
destitute  of  the  power  of  photosynthesis  that  they  must  rob 
other  plants  of  all  needed  food  or  die  of  starvation.    Some,  like 
the  cancer  root  (Aphyllon)  are  root  parasites ;  others,  like  the 
dodder  (Fig.  16),  are  parasitic  on  stems  above  ground.    The 
most  dependent  species  of  all,  such  as  the  flax  dodder,  can  live 
on  only  one  kind  of  host,  while  the  coarse  orange-stemmed 

1  See  Bergen,  Flora  of  the  Northeastern  States. 

2  Alectorolophus  hirsutus. 


SAPROPHYTES  AND  CARNIVOROUS  PLANTS        409 


dodder,1  which  is  common  all  over  the  central  and  the  north- 
eastern states,  grows  freely  on  many  kinds  of  plants,  from 
golden-rods  to  willows. 

381.  Saprophytes.    A  saprophyte  (meaning  decay  plant)  is 
a  plant  of  which  the  nutrition  is  largely  or  wholly  dependent 
on  the  absorption  of  organic 

material,  usually  when  in  a 
state  of  fermentation  or  de- 
cay. Most  plants  of  this  kind 
are  fungi  (Chapter  xxn),  but 
there  are  a  few  saprophytic 
seed  plants,  the  Indian  pipe, 
so  common  in  coniferous 
woods,  being  one  of  the 
most  familiar.  In  appear- 
ance the  saprophytes  re- 
semble parasites  so  far  as 
the  absence  of  green  color 
is  concerned  and  of  course 
they  do  little  or  no  photo- 
synthetic  work. 

382.  Carnivorous  plants. 
In  the  ordinary  pitcher 
plants  (Fig.   311)  the  leaf 
appears  in  the  shape  of  a 
more  or  less  hooded  pitcher. 

These  pitchers  are  usually          FIG.  311.  Common  pitcher  plant 
partly  filled  with  water,  and  (Sarracenia  purpurea) 

in  this  water  very  many  At  the  ^-^^,Hke  leaves  is 
drowned  and  decaying 

insects  are  commonly  to  be  found.  The  insects  have  flown  or 
crawled  into  the  pitcher,  and,  once  inside,  have  been  unable 
to  escape  on  account  of  the  dense  growth  of  bristly  hairs  about 
the  mouth,  all  pointing  inward  and  downward.  How  much  the 
,l  Cusruta  Gronovii. 


410 


PARASITES  AND  CARNIVOROUS  PLANTS 


*r 


f 


FIG.  312.   Sundew  (Drosera  rotundifolia) 

common  American  pitcher  plants  depend  for  nourishment  on 
the  drowned  insects  in  the  pitchers  is  not  definitely  known,  but 
it  is  certain  that  some  of  the  tropical  species  require  such  food.1 

1  Where  the  Sarracenia  is  abundant  it  will  be  found  interesting  and  profit- 
able to  make  a  careful  class  study  of  its  leaves.  See  Geddes,  Chapters  in 
Modern  Botany,  Chapters  i  and  n. 


SUNDEWS  AND  VENUS  FLYTRAP  411 

In  other  rather  common  plants,  the  sundews,  insects  are 
caught  by  a  sticky  secretion  which  proceeds  from  hairs  on  the 
leaves.  In  one  of  the  commonest  sundews  (Fig.  312)  the  leaves 
consist  of  a  roundish  blade,  borne  on  a  moderately  long  petiole. 
On  the  inner  surface  and  round  the  margin  of  the  blade  are 
borne  a  considerable  number  of  short  bristles,  each  terminating 
in  a  knob,  which  is  covered  with  a  clear,  sticky  liquid.  When  a 
small  insect  touches  one  of  the  sticky  knobs  it  is  held  fast,  and 
the  hairs  at  once  begin  to 
close  over  it,  as  shown  in 
Fig.  313.  Here  it  soon 
dies  and  then  usually  re- 
mains  for  many  days, 
while  the  leaf  pours  out 
a  juice  by  which  the 
soluble  parts  of  the  insect . 
are  digested.  The  liquid 
containing  the  digested  ];  ':'|  f- 

portions  is  then  absorbed 

: ,         .       ,      „  .,  FIG.  313.   Leaves  of  sundew 

by  the  leaf  and  contrib- 

„    The  one  at  the  left  has  all  its  tentacles  closed 

utes  an  important  part  of      over  captured  prey .  the  one  at  the  right  has 

the    nourishment    of    the       only  half  of  them  thus  closed.     Somewhat 
T  i  -i     j_i  T  i        magnified.  —  After  Darwin 

plant,  while  the  undigested 

fragments,  such  as  legs,  wing  cases,  and  so  on,  remain  on  the 
surface  of  the  leaf  or  may  drop  off  after  the  hairs  let  go  their 
hold  on  the  captive  insect. 

In  the  Venus  flytrap,  which  grows  in  the  sandy  regions  of 
eastern  North  Carolina,  the  mechanism  for  catching  insects  is 
still  more  remarkable.  The  leaves,  as  shown  in  Fig.  314,  termi- 
nate in  a  hinged  portion,  which  is  surrounded  by  a  fringe  of 
stiff  bristles.  On  the  inside  of  each  half  of  the  trap  grow  three 
short  hairs.  The  trap  is  so  sensitive  that  when  these  hairs  are 
touched  it  closes  with  a  jerk  and  very  generally  succeeds  in 
capturing  the  fly  or  other  insect  which  has  sprung  it.  The 
imprisoned  insect  tlien-'dies  and  is  digested,  somewhat  as  in  the 


412 


PARASITES  AND  CARNIVOROUS  PLANTS 


case  of  those  caught  by  the  sundew,  after  which  the  trap  reopens 
and  is  ready  for  fresh  captures. 

383.  Object  of  catching  animal  food.  It  is  easy  to  under- 
stand why  a  good  many  kinds  of  plants  have  taken  to  catch- 
ing insects  and  absorbing  the  digested  products.  Carnivorous, 
or  flesh-eating,  plants  belong  usually  to  one  of  two  classes  as 


FIG.  314.   Venus  flytrap  (Dioncea  muscipula) 

regards  their  place  of  growth ;  they  are  bog  plants  or  air  plants. 
In  either  case  their  roots  find  it  difficult  to  secure  much  nitrogen- 
containing  food, — that  is,  much  food  out  of  which  proteid  mate- 
rial can  be  built  up.  Animal  food,  being  itself  largely  proteid,  is 
admirably  adapted  to  nourish  the  growing  parts  of  plants,  and 
those  which  could  develop  insect-catching  powers  would  stand 
a  far  better  chance  to  exist  as  air  plants  or  in  the  thin,  watery 
soil  of  bogs  than  plants  which  had  acquired  no  such  resources. 


CHAPTEK  XXXI 
HOW  PLANTS  PROTECT  THEMSELVES  FROM  ANIMALS 

384.  Destruction  by  animals     All  animals  are  supported 
directly   or  indirectly   by   plants.    In  some  cases  the  animal 
secures  its  food  without  much  damage  to  the  plant  on  which  it 
feeds.    Browsing  on  the  lower  branches  of  a  tree  may  do  it  little 
injury,  and  grazing  animals,  if  not  numerous,  may  not  seriously 
harm  the  pasture  in  which    they  feed.    Fruit-eating  animals 
may  even  be  of  much  service  by  dispersing  seeds  (Sec.  420). 
But  seed-eating  birds  and  quadrupeds,  animals  which  (like  the 
hog)  dig  up  fleshy  roots,  rootstocks,  tubers,  or  bulbs,  and  eat 
them,  or  animals  which  (like  the  sheep)  graze  so  closely  as  to 
expose  the  roots  of  grasses  or  even  of  forest  trees  to  be  parched 
by  the  sun,  destroy  immense  numbers  of  plants.    Many  trees, 
as  the  apple,  peach,  and  black  locust,  have  the  trunk  fatally 
weakened  by  the  boring  larvae  of  insects.     Leaf-eating  insects, 
such  as  grasshoppers  and  caterpillars,  cause  immense  damage 
to  foliage,  and  others,  like  the  chinch  bug  so  destructive  to 
grain  crops,  suck  the  juices  from  roots,  stems,  or  leaves. 

385.  Some  modes  of  protection  from  animals.    Many  of  the 
characteristics  of  plants  may  be  wholly  or  partly  due  to  adapta- 
tions for  protective  purposes,  while  in  particular  cases  we  cannot 
be  sure  of  the  fact.    Perching  on  lofty  rocks  or  on  branches  of 
trees,  burying  the  perennial  part  (bulb,  rootstock,  etc.)  under- 
ground, growing  in  dense  masses,  like  a  canebrake  or  a  thicket 
of  blackberry  bushes,  —  all  such  habits  of  plants  may  be  partly 
or  altogether  valuable  to  the  plant  as  means  of  avoiding  the 
attacks  of  animals,  but  this  cannot  be  proved.    On  the  other 
hand,  there  are  plenty  of  instances  of  structures,  habits,  or  accu- 
mulations of  stored  material  in  their  tissue  which  plants  seem 

413 


414 


HOW  PLANTS  PROTECT  THEMSELVES 


to  have  acquired  mainly  or  entirely  as  means  of  defense.    Some 
of  the  most  important  are : 

1.  The  habit  of  keeping  a  bodyguard  of  ants. 

2.  Forming  tough,    corky,  woody,   limy,  or    flinty,   and    therefore 

nearly  uneatable,  tissue. 

3.  Arming  exposed  parts  with  cutting  edges,  sharp  or  stinging  hairs, 

prickles,  or  thorns. 

4.  Accumulating  unpleasant   or    poisonous    substances    in    exposed 

parts. 

386.  Ant  plants.  Some  ants  live  on  vegetable  food,  but  most 
of  them  eat  only  animal  food,  and  these  latter  are  extremely 
voracious.  It  has  been  estimated  by  a  careful  scientist,  an 


FIG.  315.    An  ant  plant  (Acacia  sphcerocephala) 

t,  thorns;  h,  hole  in  thorn;  n,  nectary;  6,  Belt's  body  on  tip  of  leaflet.  —  After 

Schimper 

authority  on  this  subject,  that  the  ants  of  a  single  nest  some- 
times destroy  as  many  as  one  hundred  thousand  insects  in  a 
day.  The  Chinese  orange  growers  in  the  province  of  Canton 
have  found  how  useful  ants  may  be  as  destroyers  of  other 
insects,  and  so  they  place  ant  nests  in  the  orange  trees  and 
extend  bamboos  across  from  one  tree  to  another,  to  serve  as 
bridges  for  the  ants  to  travel  on. 


ANT  PLANTS;  UNEATABLE  PLANTS  415 

Certain  tropical  trees  offer  ants  special  inducements  to  estab- 
lish colonies  on  their  trunks  and  'branches.  The  attractions 
which  are  offered  to  ants  by  various  kinds  of  trees  differ  greatly. 
One  of  the  most  interesting  adaptations  is  that  of  an  acacia 
(Fig.  315),  which  furnishes  little  growths  at  the  ends  of  the 
leaflets  which  serve  as  ant  food.  These  little  growths  are  known 
from  their  discoverer  as  Belt's  bodies.  The  ants  bore  holes  into 
the  large,  hollow,  stipular  thorns  shown  in  the  figure,  live  in 
these  thorns,  feed  on  the  Belt's  bodies,  and  protect  the  acacia 
from  insect  and  other  enemies.  A  nectary  on  the  leaf  furnishes 
additional  food  to  the  ant  inhabitants  of  the  tree.  A  great  multi- 
tude of  plants,  some  of  them  herbs,  offer  more  or  less  impor- 
tant inducements  to  attract  ant  visitors ;  the  species  which  are 
known  to  do  this  number  over  three  thousand.1 

387.  Plants  of  uneatable  texture.  Whenever  tender  and 
juicy  herbage  is  to  be  had,  plants  of  hard  and  stringy  texture 
are  left  untouched  by  grazing  animals.  The  flinty-stemmed 
horsetails  (Equisetum,  Sec.  316)  and  the  dry,  tough  rushes  are 
familiar  examples  of  uneatable  plants  of  damp  soil.  In  pastures 
there  grow  such  perennials  as  the  bracken  fern  and  the  hard- 
hack  of  New  England  and  the  ironweed  and  vervains  of  the 
central  states,  which  are  so  harsh  and  woody  that  the  hungriest 
browsing  animal  is  rarely,  if  ever,  seen  to  molest  them.  Still 
other  plants,  like  the  knotgrass  and  cinquefoil  of  our  dooryards, 
are  doubly  safe,  from  their  growing  so  close  to  the  ground  as  to 
be  hard  to  graze,  and  from  their  woody  and  unpalatable  nature. 
The  date  palm,  which  can  easily  be  raised  from  the  seed  in  the 
botanical  laboratory,  is  an  excellent  instance  of  the  same  un- 
eatable quality,  found  in  a  tropical  or  sub-tropical  plant.  Other 
good  examples  are  the  shrubs  of  heath  lands  and  of  such  coria- 
ceous, or  leathery-leafed,  thickets  as  the  Australian  scrub  and 
the  California  chaparral. 

1  Possibly  in  many  cases  the  attractiveness  of  plants  for  ants  is  only 
incidental  and  has  not  been  evolved  with  direct  reference  to  the  protection 
to  be  rendered  by  these  insects. 


416 


HOW  PLANTS  PROTECT  THEMSELVES 


388.  Plants  with  weapons  for  defense.1  Multitudes  of  plants, 
which  might  otherwise  have  been  subject  to  the  attacks  of  graz- 
ing or  browsing  animals,  have  acquired  what 
have  with  reason  been  called  weapons. 
Shrubs  and  trees  not  infrequently  produce 
sharp-pointed  branches,  familiar  in  our  own 
crab  apple,  wild  plum,  thorn  trees,  and, 
above  all,  in  the  honey  locust  (Fig.  35), 


FIG.  316.   Spiny  leaves  of  barberry 

whose  formidable  thorns  often  branch  in  a  very  complicated 
manner.    It  is  noteworthy  that  the  protection  given  by  thorns 

is  not  from  those  of  the 
season,  but  from  the  dry 
and  hard  ones  of  preceding 
years. 

Leaves  modified  into 
thorns  are  very  perfectly 
exemplified  in  the  barberry 
(Fig.  316).  It  is  much  com- 
moner, however,  to  find  the 
leaf  extending  its  midrib  or 

its  veins  out  into  sPin7  Points> 
as  the  thistle  does,  or  bearing 

spines  or  prickles  on  its  midrib,  as  is  the  case  with  some  night- 
shades (Fig.  317)  and  with  so  many  roses. 

1  See  Kerner  and  Oliver,  Natural  History  of  Plants,  Vol.  I,  p.  430. 


FIG.  317.  Spiny  leaf  of  a 
nightshade  (Solarium 
atropurpureum) 


PRICKLES,  THORNS,  AND  STINGING  HAIRS         417 


Prickles,  which  are  merely  hard,  sharp-pointed  projections 
from  the  epidermis,  are  of  too  common  occurrence  to  need 
illustration. 

Thorns  are  often  found  to  be  modified  stipules,  and  in  our 
common  locust  (Fig.  319)  the  bud,  or  the  very  young  shoot 


stipules  of 
locust 


FIG.  318.  Euphorbia  splendens 

The  spines  are  dead  and 
dry  stipules 

which  proceeds  from  it,  is  admi- 
rably protected  by  the  jutting  thorn 
of  the  previous  year  on  either  side. 

389.  Pointed,  barbed,  and  sting-  FIG.  319.  Thorn 
ing  hairs.  On  many  plants  needle- 
pointed  hairs  form  efficient  defensive  weapons. 
Sometimes  these  hairs  are  roughened,  like  those  of  the  bugloss 
(Fig.  320, 6) ;  sometimes  they  are  decidedly  barbed.  If  the  barbs 
are  well  developed,  as  they  are  in  the  small  but  formidable 
bristles  of  prickly  pear  cactuses,  they  may  cause  the  hairs  to 
travel  far  into  the  flesh  of  animals  and  cause  intense  pain.  In 
the  nettle  (Fig.  320,  a)  the  hairs  are  efficient  stings,  with  a 
brittle  tip,  which  on  breaking  off  exposes  a  sharp,  jagged  tube 
full  of  irritating  fluid.  These  tubular  hairs,  with  their  poisonous 
contents,  will  be  found  sticking  in  the  skin  of  the  hand  or  the 
face  after  incautious  contact  with  nettles,  and  the  violent  itch- 
ing which  follows  is  only  too  familiar  to  most  people. 


418 


HOW  PLANTS   PROTECT   THEMSELVES 


390.  Cutting  leaves.  Some  grasses  and  sedges  are  generally 
avoided  by  cattle  because  of  the  sharp,  cutting  edges  of  their 
leaves,  which  will  readily  slit  the  skin  of  one's  hand  if  they  are 
drawn  rapidly  through  the  ringers.  Under  the  microscope  the 
margins  of  such  leaves  are  seen  to  be  regularly  and  thickly  set 
with  sharp  teeth  like  those  of  a  saw  (Fig.  320,  c,  d). 

a 


EIG.  320.   Stinging  hairs  and  cutting  leaves 

«,  stinging  hairs  on  leaf  of  nettle;  b,  bristle  of  the  bugloss;  c,  barbed  margin  of 
a  leaf  of  sedge;  d,  barbed  margin  of  a  leaf  of  grass.  All  much  magnified.  — 
After  Kerner 

391.  Weapons  of  desert  plants.  In  temperate  regions,  where 
vegetation  is  usually  abundant,  such  moderate  means  of  protec- 
tion as  have  just  been  described  are  generally  sufficient  to  insure 
the  safety  of  the  plants  which  have  developed  them.  But  in 
desert  or  semi-desert  regions  the  extreme  scarcity  of  plant  life 
especially  exposes  the  few  plants  that  occur  there  to  the  attacks 


OFFENSIVE  OR  POISONOUS  PLANTS  419 

of  herbivorous  animals.  Accordingly,  great  numbers  of  desert 
plants  are  characterized  by  nauseating  or  poisonous  qualities  or 
by  the  presence  of  astonishingly  developed  thorns  (Figs.  50, 
357),  while  some  combine  both  of  these  means  of  defense. 

392.  Offensive  or  poisonous  plants.  A  disgusting  smell  is 
one  of  the  common  safeguards  which  keep  .plants  from  being 
eaten.  The  dog  fennel  (Fig.  364),  the  hound's  tongue  (Cyno- 
glossum),  the  Martynia,  and  the  tomato  plant  are  common  exam- 
ples of  rank-smelling  plants  which  are  offensive  to  most  grazing 
animals  and  so  are  let  alone  by  them.  Oftentimes,  as  in  the 
case  of  the  jiinson  weed  (Datura),  the  tobacco  plant,  and  the 
poisonous  hemlock  (Conium),  the  smell  serves  as  a  warning  of 
the  poisonous  nature  of  the  plant. 

A  bitter,  nauseating,  or  biting  taste  protects  many  plants  from 
destruction  by  animals.  Buckeye,  horse-chestnut,  and  buck- 
thorn twigs  and  leaves  are  so  bitter  that  browsing  animals  and 
most  insects  let  them  alone.  Tansy,  ragweed,  boneset,  southern- 
wood, and  wormwood  are  safe  for  the  same  reason.  The  nau- 
seous taste  of  many  kinds  of  leaves  and  stems,  such  as  those  of 
the  potato,  and  the  fiery  taste  of  peppercorns,  red  peppers,  mus- 
tard, and  horse-radish,  make  these  substances  uneatable  for  most 
animals.  Probably  both  the  smell  and  the  taste  of  onions  serve 
to  insure  the  safety  of  the  bulbs  from  the  attacks  of  most  grubs, 
and  the  hard  corm  of  the  jack-in-the-pulpit  (Arisamia)  is  care- 
fully let  alone  on  account  of  the  blistering  nature  of  its  contents. 

Poisonous  plants  are  usually  shunned  by  grown-up  animals 
unless  they  are  famished,  though  the  young  ones  will  some- 
times eat  such  plants  and  may  be  killed  by  them.  Almost  any 
part  of  a  poisonous  species  may  contain  the  poison  character- 
istic of  the  plant,  but,  for  obvious  reasons,  such  substances  are 
especially  apt  to  be  stored  in  the  parts  of  the  plant  where  its 
supply  of  reserve  food  is  kept. 


CHAPTER  XXXII 
POLLINATION  OF  FLOWERS  AND  PROTECTION  OF  POLLEN 

393.  Topics  of  the  Chapter.    The  ecology  of  flowers  is  con- 
cerned mainly  with  the  means  by  which  the  transference  of 
pollen,  or  pollination,  is  effected,  and  with  the  ways  in  which 
pollen  is  kept  away  from  undesirable  insect  visitors  and  from 
rain. 

394.  Cross  pollination  and  self  pollination.    It  was  long  sup- 
posed by  botanists  that  the  pollen  of  any  bisexual  flower  needed 
only  to  be  placed  on  the  stigma  of  the  same  flower  to  insure  sat- 
isfactory fertilization.    But  in  1857  and  1858  the  great  English 
naturalist,  Charles  Darwin,  stated  that  certain  kinds  of  flowers 
were  entirely  dependent  for  fertilization  on  the  transference 
of  pollen  from  one  plant  to  another.    It  was  also  shown  that 
probably  nearly  all  attractive  flowers,  even  if  they  can  produce 
some  seed  when  self-pollinated,  do  far  better  when  pollinated 
from  the  flowers  of  another  plant  of  the  same  kind.1    This  im- 
portant fact  was  established  by  a  long  series  of  experiments 
on  the  number  and  vitality  of  seeds  produced  by  a  flower  when 
treated  with  its  own  pollen,  or  self -pollinated,  and  when  treated 
with  pollen  from  another  flower  of  the  same  kind,  or  cross- 
pollinated? 

Another  important  advantage  of  cross  pollination  is  that  it 
tends  to  give  the  offspring  additional  variability  (Chapter  XL), 
and  thus  enables  them  better  to  adapt  themselves  to  changing 
environment  or  to  any  difficult  conditions. 

1  See   Darwin,   Cross  and  Self  Fertilization  in  the  Vegetable  Kingdom 
(especially  Chapters  i  and  n). 

2  On  dispersion  of  pollen,  see  Kernel*  and  Oliver,  Natural  History  of 
Plants,  Vol.  II,  pp.  129-287. 

420 


WIND-POLLINATED  FLOWERS  421 

It  should  always  be  kept  in  mind  that  many  of  the  most 
successful  plants,  including  a  large  number  of  troublesome 
weeds,  are  capable  of  self  pollination. 

395.  Wind-pollinated  flowers.1  It  has  already  been  mentioned 
that  some  pollen  is  dry  and  powdery,  and  other  kinds  are  more 
or  less  sticky.  Pollen  of  the  dusty  sort  is  light,  and  therefore 
adapted  to  be  blown  about  by  the  wind.  Any  one  who  has  been 
much  in  cornfields  after  the  corn  has  "  tasseled  "  has  noticed  the 
pale  yellow,  dusty  pollen  which  flies  about  when  a  cornstalk  is 
jostled,  and  which  collects  in  considerable  quantities  on  the 
blades  of  the  leaves.  Corn  is  monoecious,  but  fertilization  is 
best  accomplished  by  pollen  blown  from  the  "  tassel "  (stamens) 
of  one  plant  being  carried  to  the  "  silk  "  (stigma  and  style  of 
the  pistils)  of  another  plant.  This  is  well  shown  by  the  fact, 
familiar  to  every  observing  farmer's  boy,  that  solitary  cornstalks, 
such  as  often  grow  very  luxuriantly  in  an  unused  barnyard  or 
similar  locality,  bear  very  imperfect  ears  or  none  at  all.  The 
common  ragweed  is  remarkable  for  the  great  quantities  of 
pollen  which  shake  off  on  to  the  shoes  or  clothes  of  the 
passer-by,  and  it  is  wind-pollinated.  So, 
too,  are  the  pines,  and  these  produce  so 
much  pollen  that  it  has  been  mistaken 
for  showers  of  sulphur,  falling  often  at 
long  distances  from  the  forests  where  it  FIG.  321.  Pistil  of  a  grass, 
was  produced.  The  pistil  of  wind-polli-  provided  with  a  feath- 

,    -,  n  f^        P     ,1  i  ^i  ery  stigma,  adapted  for 

nated  flowers  is  often  feathery  and  thus       wind_pollh;ation 

adapted    to    catch    flyinsr    pollen    grains 

r  After  Thome 

(Fig.  321).    Other  characteristics  of 

such  flowers  are  the  inconspicuous  character  of  their  perianth, 
which  is  usually  green  or  greenish,  the  absence  of  odor  and 
of  nectar,  the  regularity  of  the  corolla,  and  the  development 
of  the  flowers  before  the  leaves  or  their  occurrence  on  stalks 
raised  above  the  leaves. 

1  See  Miss  Newell,  Botany  Reader,  Part  II,  Chapter  vn. 


422 


POLLINATION  OF   FLOWERS 


Pollen  is,  in  the  case  of  a  few  aquatic  plants,  carried  from 
flower  to  flower 'by  the  water  in  which  the  plant  grows. 

396.  Insect-pollinated  flowers.  Most  plants  which  require 
cross  pollination  depend  upon  insects  as  pollen  carriers,1  and  it 
may  be  stated  as  a  general  fact  that  the  showy  colors  and  mark- 
ings of  flowers  and  their  odors  all  serve  as  so  many  advertise- 
ments of  the  nectar  (commonly  but  wrongly  called  honey)  or  of 
the  nourishing  pollen  which  the  flower  has  to  offer  to  insect 
visitors. 

Many  insects  depend  mainly  or  wholly  upon  the  nectar  and 
the  pollen  of  flowers  for  their  food.  Such  insects  usually  visit 
during  any  given  trip  only  one  kind  of  flower,  and  therefore 
carry  but  one  kind  of  pollen.  Going  straight  from  one  flower  to 
another  with  this,  they  evidently  waste  far  less  pollen  than  the 

wind  or  water  must  waste.  It  is 
therefore  clearly  advantageous  to 
flowers  to  develop  such  adapta- 
tions as  fit  them  to  attract  insect 
visitors,  and  to  give  pollen  to  the 
latter  and  receive  it  from  them. 
397.  Pollen-carrying  appara- 
tus of  insects.2  Ants  and  some 
beetles  which  visit  flowers  have 
smooth  bodies,  to  which  little 
FIG.  322  ^  pollen  adheres,  so  that  their  visits 

A,  right  hind  leg  of  a  honeybee  (seen  are  often  of   slight  value   to   the 

from   behind  and  within);    B    the  fl              b    t                beetles;  all  but- 

tibia;    ti,    seen    from    the    outside,  * 

showing  the  collecting  basket  formed  terflies    and   moths,   and   most 

of  stiff  hairs.  -  After  Muller  ^  ^ye  bodieg  roughened  with 

scales  or  hairs,  which  hold  a  good  deal  of  pollen  entangled.  In 
the  common  honeybee  (and  in  many  other  kinds)  the  greater 
part  of  the  insect  is  hairy,  and  there  are  special  collecting 
baskets,  formed  by  bristle-like  hairs,  on  the  hind  legs  (Fig.  322). 

1  A  few  are  pollinated  by  snails  ;  many  more  by  humming  birds  and  other 
birds.  2  See  P.  Kimth,  Handbuch  der  Bluthenbiologie,  Vol.  I. 


-ti 


ATTRACTIONS  FOR  INSECTS  423 

It  is  easy  to  see  the  load  of  pollen  accumulated  in  these  baskets 
after  such  a  bee  has  visited  several  flowers.  Of  course  the  pollen 
which  the  bee  packs  in  the  baskets  and  carries  off  to  the  hive, 
to  be  stored  for  food,  is  of  no  use  in  pollination.  In  fact,  such 
pollen  is  in  one  sense  entirely  wasted.  But  since  such  bees  as 
have  these  collecting  baskets  are  the  most  industrious  visitors 
to  flowers,  they  accomplish  an  immense  share  of  the  work  of 
pollination  by  means  of  the  pollen  grains,  which  stick  to  their 
hairy  coats  and  are  then  transferred  to 
other  flowers  of  the  same  kind  next  visited 
by  the  bee. 

398.  Nectar  and  nectaries.    Nectar  is  a 
sweet  liquid   which    flowers    secrete    and 

which  attracts  insects.    After  partial  diges-  VYV — Y-£?— -9 

tion  in  the  crop  of  the  bee,  nectar  be- 
comes honey.  Those  flowers  which  secrete 
nectar  usually  do  so  by  means  of  nectar 

glands,  small   organs   situated  often   near  FlG:  3f '  Stamens  and 

pistil    of    the    grape 
the  base  of  the  flower,  as  shown  in  Fig.  323.      (magnified),  with  a 

Sometimes  the  nectar  clings  in  droplets  to      nectar   gland  g  be- 
the   surface  of  the  nectar  glands ;    some-      tween  the  stamens 
times    it   is    stored    in   little    cavities    or         After  Decaisne 
pouches  called  nectaries.    The  pouches  at  the  bases  of  columbine 
petals  are  among  the  most  familiar  of  nectaries. 

399.  Odors  of  flowers.    The  acuteness  of  the  sense  of  smell 
among  insects  is  a  familiar  fact.    Flies  buzz   about   the  wire 
netting  which  covers  a  piece  of  fresh  meat  or  a  dish  of  sirup, 
and  bees,  wasps,  and  hornets  will  fairly  besiege  the  window 
screens    of  a  kitchen   where    preserving   is  going    on.     Many 
plants  find  it  possible  to  attract  as  many  insect  visitors  as  they 
need  without  giving  off  any  scent  perceptible  to  us,  but  small 
flowers,  like  the  mignonette,  and  night-blooming  ones,  like  the 
white  tobacco  and  the  evening  primrose,  are  sweet-scented  to 
attract  night-flying  moths.     It  is  interesting   to   observe   that 
the  majority  of  the  flowers  which  bloom  at  night  are  white  or 


424  POLLINATION  OF  FLOWERS 

yellow,  and  that  they  are  much  more  generally  sweet-scented 
than  flowers  which  bloom  during  the  day.  Many  are  odorous 
during  only  a  few  hours  of  the  twenty-four,  just  at  the  time 
when  the  special  insects  which  pollinate  them  are  on  the  wing. 
A  few  flowers  (purplish,  brownish,  or  greenish  colored)  are  car- 
rion-scented and  attract  flies. 

400.  Colors  of  flowers.  Flowers  which  are  of  any  other  color 
than  green  probably  in  most  cases  display  their  colors  to  attract 
insects,  or  occasionally  birds.  The  principal  color  of  the  flower 
is  most  frequently  due  to  showy  petals;  sometimes,  as  in  the 
anemone  and  marsh  marigold,  it  belongs  to  the  sepals ;  and  not 
infrequently,  as  in  some  cornels  (Frontispiece)  and  Euphorbias 
(Fig.  318),  the  involucre  is  more  brilliant  and  conspicuous  than 
any  part  of  the  flower  strictly  so  called.  In  the  willows  and 
chestnuts  the  stamens  are  the  conspicuous  parts. 

Different  kinds  of  insects  appear  to  be  especially  attracted 
by  different  colors.  In  general,  dull  yellow,  brownish,  or  dark 
purple  flowers,  especially  if  small,  seem  to  depend  largely  on 
the  visits  of  flies.  Eed,  violet,  and  blue  are  the  colors  by  v/hich 
bees  and  butterflies  are  most  readily  enticed.  The  power  of 
bees  to  distinguish  colors  has  been  shown  by  a  most  interesting 
set  of  experiments  in  which  daubs  of  honey  were  put  on  slips 
of  glass  laid  on  separate  pieces  of  paper,  each  of  a  different 
color,  and  exposed  where  bees  would  find  them.1 

It  is  certain,  however,  that  colors  are  less  important  means 
of  attraction  than  odors  from  the  fact  that  insects  are  extremely 
near-sighted.  Butterflies  and  moths  cannot  see  distinctly  at  a 
distance  of  more  than  about  five  feet,  bees  and  wasps  at  more 
than  two  feet,  and  flies  at  more  than  two  and  a  fourth  feet. 
Probably  no  insects  can  make  out  objects  clearly  more  than 
six  feet  away.2  Yet  it  is  quite  possible  that  their  attention  is 
attracted  by  colors  at  distances  greater  than  those  mentioned. 

1  See  Lubbock,  Flowers,  Fruits,  and  Leaves,  Chapter  i.  On  the  general 
subject  of  colors  and  odors  in  relation  to  insects,  see  P.  Knuth,  Handbuch 
der  Bliitheribiologie.  2  See  Packard,  Text-Book  of  Entomology,  p.  260. 


FACILITIES  FOR  INSECT  VISITS  425 

401.  Facilities  for  insect  visits.  Regular  flowers  with  radial 
symmetry  usually  have  no  special  adaptations  to  make  them 
singly  accessible  to  insects,  but  lie  open  to  all  comers.  They 
do,  however,  make  themselves  much  more  attractive  and  afford 
especial  inducements  in  the  matter  of  saving  time  to  flower- 
frequenting  insects  by  being  grouped.  This  purpose  is  undoubt- 
edly served  by  dense  flower  clusters,  such  as  those  of  the  lilac, 
the  phlox,  and  the  elder,  and  especially  by  heads  like  those  of 
the  button  bush  (Ceplialanthus)  and  by  the  peculiar  form  of 
head  found  in  so-called  composite  flowers,  like  the  sunflower, 
the  bachelor's  button,  and  the  yarrow 
(Fig.  144).  In  many  such  clusters  the 
flowers  are  specialized,  some  carrying 
a  showy  strap-shaped  corolla,  to  serve 
as  an  advertisement  of  the  nectar  and 
pollen  contained  in  the  inconspicuous 
tubular  flowers.  Flowers  with  bilateral 
symmetry  probably  always  are  more 
or  less  adapted  to  particular  insect  (or 
other)  visitors.  The  adaptations  are  FJG  324  A  beetle  on  the 
extremely  numerous  ;  here  only  a  very  flower  of  the  twayblade 
few  of  the  simpler  ones  will  be  pointed  slightly  enlarged.— After 
out.  Where  there  is  a  drooping  lower  Behrens 

petal  or,  in  the  case  of  a  sympetalous  corolla,  a  lower  lip, 
this  serves  as  a  perch  upon  which  flying  insects  may  alight  and 
stand  while  they  explore  the  flower,  as  the  beetle  is  doing  in 
Fig.  324.  In  Fig.  325  one  bumblebee  stands  with  her  legs 
partially  encircling  the  lower  lip  of  the  dead-nettle  flower,  while 
another  perches  on  the  sort  of  grating  made  by  the  stamens 
of  the  horse-chestnut  flower.  The  honeybee  entering  the  violet 
clings  to  the  beautifully  bearded  portion  of  the  two  lateral 
petals,  while  she  sucks  the  nectar  from  the  spur  beneath.  All 
bilaterally  symmetrical  flowers  seem  to  be  specially  adapted  to 
compel  visiting  insects  to  enter  them  in  the  best  way  to 
secure  transference  of  -pollen. 


426 


POLLINATION  OF  FLOWERS 


402.  Protection  of   pollen  from  unwelcome  visitors.    It  is 

usually  desirable  for  the  flower  to  prevent  the  entrance  of 
small  creeping  insects,  such  as  ants,  which  carry  little  pollen 
and  eat  a  relatively  large  amount  of  it.  The  means  adopted  to 
secure  this  result  are  many  and  curious.  In  some  plants,  as 
the  common  catchfly,  there  is  a  sticky  ring  about  the  peduncle, 
some  distance  below  the  flowers,  and  this  forms  an  effectual 
barrier  against  ants  and  like  insects.  In  a  few  plants,  as  the 


FIG.  325.   Bees  visiting  flowers 


At  the  left,  a  bumblebee  on  the  flower  of  the  dead  nettle;  below,  a  similar  bee  in 
the  flower  of  the  horse-chestnut ;  above,  a  honeybee  in  the  flower  of  a  violet. 
Modified.  —  After  Behrens 

teasel  and  the  cup  plant  (Silphium  perfoliatum),  rain  water  col- 
lects at  the  junctions  of  the  leaves  with  the  stem  and  forms 
an  effectual  barrier  against  creeping  insects.  Very  frequently 
the  calyx  tube  is  covered  with  hairs,  which  are  sometimes 
sticky.  How  these  thickets  of  hairs  may  appear  to  a  small 
insect  can  perhaps  be  realized  from  Fig.  32 6.1 

1  On  protection  of  pollen,  see  Kerner  and  Oliver,  Natural  History  of 
Plants,  Vol.  II,  pp.  95-109. 


PROTECTION  FROM  UNWELCOME  VISITORS        427 

Sometimes  the  recurved  petals  or  divisions  of  the   corolla 
stand  in  the  way  of  creeping  insects.    In  other  cases  the  throat 


FIG.  326.   Branching  hairs  from  the  outside  of  the  corolla  of  the 
common  mullein 

Magnified.  —  After  Tschirch 

of  the  corolla  is  much  narrowed  or  closed  by  hairs,  or  by  ap- 
pendages. Those  flowers  which  have  one  or  more  sepals  or 
petals  prolonged  into  spurs,  like  the  nasturtium  and  the  colum- 
bine, are  inaccessible  to  most  insects  except  those  which  have 


FIG.  327.    A  sphinx  moth,  with  a  long  sucking  tube 

a  tongue  or  a  sucking  tube  long  enough  to  reach  to  the  nectary 
at  the  bottom  of  the  spur.  The  large  sphinx  moth,  shown  in 
Fig.  327,  which  is  a  common  visitor  to  the  flowers  of  the 


428  POLLINATION  OF  FLOWERS 

evening  primrose,  is  an  example  of  an  insect  especially  adapted 
to  reach  deep  into  long  tubular  flowers. 

A  little  search  among  flowers,  such  as  those  of  the  columbine 
and  the  foxglove,  will  usually  disclose  many  which  have  had  the 
corolla  bitten  through  by  bees  which  are  unable  (or  unwilling 
to  take  the  trouble)  to  get  at  the  nectar  by  fair  means,  and 
which  therefore  steal  it. 

403.  Bird-pollinated  flowers.    Some  flowers  with  very  long 
tubular  corollas  depend  entirely  upon  birds  to  carry  their  pollen 
for  them.    Among  garden  flowers  the  gladiolus,  the  scarlet  salvia, 
the  canna,  and  the  trumpet  honeysuckle  are  largely  dependent 
upon  humming  birds  for  their  pollination.    The  wild  balsam, 
or  jewelweed,  the  swamp  thistle,  and  the  trumpet  creeper  are 
also  favorite  flowers  of  the  humming  bird. 

404.  Prevention  of  self  pollination.    Dioecious  flowers  are, 
of  course,  quite  incapable  of  self  pollination.    Pistillate  monoe- 
cious flowers  may  be  pollinated  by  staminate  ones  on  the  same 
plant,  but  this  does  not  secure  as  good  seed  as  is  secured  by 
having  pollen  brought  to  the  pistil  from  a  different  plant  of  the 
same  kind. 

In  perfect  flowers  self  pollination  would  commonly  occur  un- 
less it  were  prevented  by  the  action  of  the  essential  organs 
or  by  something  in  the  structure  of  the  flower.  In  reality, 
many  flowers  which  at  first  sight  would  appear  to  be  designed 
to  secure  self  pollination  are  almost  or  quite  incapable  of  it. 
Frequently  the  pollen  from  another  plant  of  the  same  species 
prevails  over  that  which  the  flower  may  shed  on  its  own  pistil, 
so  that  when  both  kinds  are  placed  on  the  stigma  at  the  same 
time  it  is  the  foreign  pollen  which  causes  fertilization.  But 
apart  from  this  fact  there  are  several  means  of  insuring  the 
presence  of  foreign  pollen,  and  only  that,  upon  the  stigrna,  just 
when  it  is  mature  enough  to  receive  pollen  tubes. 

405.  Stamens  and  pistils  maturing  at  different  times.    If 
the  stamens  mature  at  a  different  time  from  the  pistils,  self 
pollination  is  as  effectually  prevented  as  though  the  plant  were 


PREVENTION  OF  SELF  POLLINATION 


429 


dioecious.  This  unequal  ma- 
turing, or  dichogamy,  occurs 
in  many  kinds  of  flowers.  In 
some,  the  figwort  and  the  com- 
mon plantain  for  example,  the 
pistil  develops  before  the  sta- 
mens, but  usually  the  reverse 
is  the  case.  The  Clerodendron} 
a  tropical  African  flower  (Fig. 
328),  illustrates  in  a  most 
striking  way  the  development 
of  stamens  before  the  pistil.  B 
The  insect  visitor,  on  its  way  FlG>  328>  Flower  of  Clerodendron 
to  the  nectary,  can  hardly  fail  in  two  stages 

to  brush  against  the  protrud-    jn  A  (earlier  stage)  the  stamens  are  ma- 
ing    Stamens  of    the   flower  in        ture,  while  the  pistil  is  still  undeveloped 

and  bent  to  one  side.    In  B  (later  stage) 
its  earlier  Stage,  A,  but  it  can-       the    stamens  have  withered,  and  the 

not  deposit  any  pollen  on  the      stigmas  have  seParated>  1>eady  for  the 
J   r  reception  of  pollen.  — After  Gray 

stigmas,  which  are   imma- 
ture, shut  together,  and  tucked  aside  out  of  reach.    On  flying 
to  a  flower  in  the  later  stage  the  pollen  just  acquired  will  be 


—stiff 


A  B  C  D 

FIG.  329.    Provisions  for  cross  pollination  in  the  high  mallow 

A,  essential  organs  as  found  in  the  bud;  B,  same  in  the  staminate  stage,  the 
anthers  discharging  pollen,  pistils  immature;  C,  intermediate  stage  (stig,  the 
united  stigmas) ;  Z),  pistillate  stage,  the  stigmas  separated,  stamens  withered. 
—  After  Muller 

1%C.  Thompsonioe. 


430 


POLLINATION  OF  FLOWERS 


lodged  on  the  prominent  stigmas  and  thus  produce  the  desired 

cross  pollination. 

Closely  related  flowers  often  differ  in  their  plan  of  pollination. 

The  high  mallow  (a  plant  cultivated  for  its  purplish  flowers), 
which  has  run  wild  to  some  extent,  is  ad- 
mirably adapted  to  secure  cross  pollination, 
since  when  its  stamens  are  shedding  pollen, 
as  in  Fig.  329,  B,  the  pistils  are  incapable  of 
receiving  it,  while  when  the  pistils  are  ma- 
ture, as  in  D,  the  stamens  are  quite  withered. 
In  the  common  low  mallow  of  our  door- 
yards  and  waysides  insect  pollination  may 


FIG.  330.    Stamens 
and  pistils  of  round- 


occur,  but  if  it  does  not,  the  curling  stigmas 


leafed  mallow 

The  stigmas  curled  fina%  come  in  contact  with  the  projecting 
round  among  the  sta-  stamens  and  receive  pollen  from  them,  as  is 

mens  to  admit  of  self    .     -,.      ,     i   •      TTI-      OOA 

pollination. -After  indicated  in  Fig.  330. 

Muiier  406.  Movements  of  floral  organs  to  aid  in 

pollination.  Besides  the  slow  movements  which  the  stamens 
and  pistil  make  in  such  cases  as  those  of  the  Clerodendron  and 


FIG.  331.    Two  flowers  of  common  sage,  one  of  them  visited  by  a  bee 
After  Lubbock 

the  mallow,  already  described,  the  parts  of  the  flower  often 
admit  of  considerable  and  rather  quick  movements  that  assist 
the  insect  visitor  to  become  dusted  or  smeared  with  pollen. 


MOVEMENTS  OF  FLORAL  ORGANS 


431 


In  some  flowers  whose  stamens  perform  rapid  movements 
when  an  insect  enters,  it  is  easy  to  see  how  directly  useful  the 
motion  of  the  stamens  is  in  securing  cross  pollination.  The 
stamens  of  the  laurel  (Kalmia)  are  held  in  a  bent  position  by 
the  expanded  corolla,  and  when  liberated  by  a  touch  throw  little 
masses  of  pollen,  with  a  quick  jerk,  against  the  body  of  the 
visiting  insect.  Barberry  flowers  have  filaments  which  are  sen- 
sitive on  the  inner  side  near  the  base,  and  when  touched  make 
the  anther  spring  up  against  the  visitor  and  dust  him  with 
pollen.  The  common  garden  sage  matures  its  anthers  earlier 
than  its  stigmas.  In  Fig.  331,  A,  the  young  flower  is  seen,  vis- 
ited by  a  bee,  and  one  anther, 


a' 


an,  is  shown  pressed  closely 
against  the  side  of  the  bee's 
abdomen.  The  stigma,  st,  is 


FIG.  332.   Flower  and  stamens  of  common  sage 

A,  p,  stigma;  a,  anthers.  JB,  the  two  stamens  in  ordinary  position;  /,  filaments; 
m,  connective  (joining  anther  cells) ;  a,  a',  anther  cells.  C,  the  anthers  and 
connectives  bent  into  a  horizontal  position  by  an  insect  pushing  against  a.  — 
After  Lubbock. 

hidden  within  the  upper  lip  of  the  corolla.  In  B,  an  older 
flower,  the  anthers  have  withered  and  the  stigma  is  now  low- 
ered so  as  to  brush  against  the  body  of  any  bee  which  may 
enter.  A  little  study  of  Fig.  332  will  make  clear  the  way 
in  which  the  anthers  are  hinged,  so  that  a  bee  striking  the 
empty  or  barren  anther  lobes,  a,  knocks  the  pollen-bearing 
lobes,  a',  into  a  horizontal  position,  so  that  they  will  lie  closely 
pressed  against  both  sides  of  its  abdomen.  Many  stigmas,  as 
those  of  catalpa  and  trumpet  creeper,  close  as  soon  as  they 
are  pollinated. 


432 


POLLINATION  OF  FLOWERS 


407.  Flowers  with  stamens  and  pistils  each  of  two  lengths. 

The  flowers  of  bluets,  partridge  berry,  the  primroses,  and  a  few 
other  common  plants  secure  cross  pollination  by  having  stamens 
and  pistils  of  two  forms  (Fig.  333).  Such  flowers  are  said  to  be 
dimorphous  (of  two  forms).  In  the  short-styled  flowers,  B,  the 
anthers  are  borne  at  the  top  of  the  corolla  tube  and  the  stigma 
stands  about  halfway  up  the  tube.  In  the  long-styled  flowers, 
A,  the  stigma  is  at  the  top  of  the  tube  and  the  anthers  are  borne 

about  halfway  up.  An  insect 
pressing  its  head  into  the  throat 
of  the  corolla  of  B  would  be- 
come dusted  with  pollen,  which 
would  be  brushed  off'  on  the 
stigma  of  a  flower  like  A.  On 
leaving  a  long-styled  flower  the 
bee's  tongue  would  be  dusted 
over  with  pollen,  some  of  which 
might  readily  be  rubbed  off  on 
the  stigma  of  the  next  short- 
styled  flower  that  was  visited. 
Cross  pollination  is  insured, 
since  all  the  flowers  on  a  plant 
are  of  one  kind,  either  long- 


FIG.  333.   Dimorphous  flowers  of 
the  primrose 


short"  styled  or 

the  pollen  is  of  two  sorts,  each 
kind  sterile  on  the  stigma  of  any  flower  of  similar  form  to 
that  from  which  it  came. 

TrimorpJwus  flowers,  with  long,  medium,  and  short  styles, 
are  found  in  a  species  of  loosestrife  and  in  the  pickerel  weed 
(Pontederia)  . 

408.  Cleistogamous  flowers.  In  marked  contrast  with  such 
flowers  as  those  discussed  in  the  preceding  sections,  which  bid 
for  insect  visitors  or  expose  their  pollen  to  be  blown  about 
by  the  wind,  are  certain  flowers  which  remain  closed  even 
during  the  pollination  of  the  stigma.  These  flowers  are  called 


CLEISTOGAMOUS  FLOWERS 


433 


cleistogamous  (meaning '  with  shut-in  fertilization)  and  are  of 
course  not  cross-pollinated.  Usually  they  occur  on  plants  which 
also  bear  flowers  adapted  for  cross  pollination,  and  in  this  case 
the  closed  flowers  are  much  less  conspicuous  than  the  others,  yet 


FIG.  334.   A  violet,  with  cleistogamous  flowers 

The  structures  which  look  like  flower  buds  are  cleistogamous  flowers-in  various 
stages  of  development.  The  pods  are  the  fruit  of  similar  flowers.  The  plant 
is  represented  as  it  appears  in  late  July  or  August,  after  the  ordinary  flowers 
have  disappeared 


434 


POLLINATION  OF  FLOWERS 


they  produce  much  seed.  Every  one  knows  the  ordinary  flowers 
of  the  violet,  but  most  people  do  not  know  that  violets  very  gen- 
erally, after  the  blossoming  season  (of  their  showy  flowers)  is 
over,  produce  many  cleistogamous  flowers,  as  shown  in  Fig.  334. 


EN: 


FIG.  335.   Protection  of  pollen  from  moisture 

At  the  left  herb  Robert  and  sweet  scabious  in  sunny  weather;  at  the  right  the 
same  flowers  during  rain.  —  After  Kerner 

409.  Protection  of  pollen  from  rain.  Pollen  is  very  generally 
protected  from  being  soaked  and  spoiled  by  rain  or  dew  by  the 
natural  position  of  the  flower,  which  prevents  rain  from  entering, 
as  in  the  case  of  most  sympetalous,  nodding  flowers,  such  as  the 


PROTECTION  OF  POLLEN  FROM   RAIN  435 

lily  of  the  valley  and  the  flowers  of  the  blueberry,  huckleberry, 
wintergreen,  and  a  multitude  of  others.  Often,  in  two-lipped 
flowers,  the  anthers  are  more  or  less  completely  covered  by 
the  upper  lip  (Fig.  331).  In  the  salver-shaped  flowers,  such  as 
those  of  phlox,  the  mouth  of  the  corolla  tube  is  often  so  narrow 
that  no  rain  or  dew  can  enter  it.  Many  corollas  of  the  same 
general  type  as  that  of  the  sweet  pea  (Fig.  126)  have  the  stamens 
covered  by  certain  petals.  A  large  number  of  flowers,  such  as 
the  crocus,  rose,  pond  lily,  magnolia,  and  many  heads,  such  as 
those  of  the  dandelion,  the  chicory,  and  the  hawkvveed,  close  in 
wet  weather  and  open  in  the  sunshine.  Sometimes  the  flower 
both  changes  its  position  and  closes,  as  is  the  case  with  the 
common  cranesbill,  the  herb  Robert,  and  the  sweet  scabious 
(Fig.  335).  In  the  linden  and  the  jewelweed  the  flowers  are 
covered  by  the  foliage  leaves  of  the  plant  so  that  rain  can  hardly 
ever  enter  them. 


CHAPTER   XXXIII 
HOW   PLANTS  ARE  SCATTERED   AND   PROPAGATED 

410.  Dispersal  of  plants  by  roots  and  rootstocks.  Some 
of  the  highest  spore  plants,  as  the  ferns,  spread  freely  by  means 
of  their  creeping  rootstocks,  and  the  gardener  who  wishes 
to  get  large  strong  ferns  quickly  often  finds  it  the  easiest  plan 


to  cut  to  pieces  and 
reset  the  rootstocks 
of  a  well-estab- 
lished plant.  In 
the  walking  fern 

(Fig.  273)  the  tip 
FIG.  336.   Plant    of    a    black    v     & 

raspberry,    showing  one    of  the  frond   roots 

branch  (stolon)  with  several  and  begins  a  new 

tips  rooting  jAant.    The  student   has   learned   (in 

After  Beal  Chapters  IV   and  vi)  that  roots  and 

underground  stems  of  many  kinds  may"  serve  to  reproduce 
the  plant.  Either  roots  or  rootstocks  may  travel  considerable 
distances  horizontally  in  the  course  of  their  growth  and  then 
shoot  up  and  produce  a  new  plant,  which  later  becomes  in- 
dependent of  the  parent.  The  sedges  (Fig.  44)  are  excellent 
illustrations  of  this  process,  and  trees  like  the  common  locust 
and  the  silver-leaf  poplar  become  great  nuisances  in  the 
neighborhood  of  lawns  and  gardens  by  sending  up  sprouts 
in  many  places.  When  growing  wild,  such  trees  as  these 

436 


DISPERSAL  OF   SEED  PLANTS  BY  BRANCHES       437 

depend    largely   upon    propagating   by   the   roots  to  keep  up 
their  numbers.1 

411.  Dispersal  of  seed  plants  by  branches.  There  is  a  shrub 
of  the  honeysuckle  family,2  common  in  the  northern  woods, 
which  is  quite  generally  known  as  hobblebush,  or  witch-hobble, 
and  sometimes  as  trip-toe.  This  is  because  the  branches  take 
root  at  the  end  and  so  form  loops  which  catch  the  foot  of  the 
passer-by.  The  same  habit  of  growth  is  ^^ 
found  in  the  raspberry  bush  (Fig.  336),  in 
one  species  of  strawberry  bush  (Euonymus), 
and  in  some  other  shrubs. 
Many  herbs,  like  the 
strawberry  plant  and  the 
cinquefoil,  send  out  long 
leafless  runners  which 
root  at  intervals  and  so 
propagate  the  plant,  carry- 
ing the  younger  individ- 
uals off  to  a  considerable 
distance  from  the  parent. 

Living  branches  may 
drop  freely  from  the  tree  and  then  take  root  and  grow,  after 
having  been  blown,  or  carried  by  a  brook  or  river,  to  a  favor- 
able spot,  perhaps  hundreds  of  yards  away.  The  so-called  snap 
willows  lose  many  live  twigs  under  conditions  suitable  for  start- 
ing new  trees. 

A  slightly  different  mode  of  dispersal  from  that  of  the  rasp- 
berry is  one  in  which  buds  separate  from  the  plant  and  serve 
to  propagate  it.  In  the  bladderwort  (Fig.  337),  at  the  close  of 
the  growing  season,  the  terminal  buds  are  released  by  the  decay 
of  the  stem  and  sink  to  the  bottom  of  the  water  in  which  the 
plants  live,  there  to  remain  dormant  until  spring.  Then  each 
bud  starts  into  life  and  gives  rise  to  a  new  individual. 


FIG.  337.   A  free  branch  and  two  buds 
of  bladderwort 

After  Beal 


1  See  Beal,  Seed  Dispersal,  Chapters  n  and  in. 

2  Viburnum  Iqntanoides. 


438 


HOW  PLANTS  ARE  SCATTERED 


412.  Dispersal  of  seed  plants  by  bulblets.  Almost  every 
farmer's  boy  knows  what  "  onion  sets  "  are.  This  name  is  often 
given  to  little  bulbs  produced  at  the  top  of  a  naked  flower  stalk, 
or  scape,  by  some  kinds  of  onions  which  do  not  usually  flower 
or  bear  seed.  Tiger  lilies  produce  somewhat  similar  bulblets  in 
the  axils  of  the  leaves,  and  there  is  a  large  number  of  species, 


FIG.  338.   Fruit  of  smoke  tree  (Rhus  Cotinus) 

Only  one  pedicel  bears  a  fruit,  all  the  others  are  sterile,  branched,  and  covered 
with  plumy  hairs 

scattered  among  numerous  families  of  plants,  all  characterized 
by  the  habit  of  producing  bulblets  or  fleshy  buds,  borne  on  the 
stems  or  leaves  above  ground  and  of  use  in  propagation.  When 
mature  the  bulblets  fall  off  readily,  and  if  they  find  lodgment 
on  unoccupied  soil  they  grow  readily  into  new  plants.  Some- 
times they  are  carried  moderate  distances  by  wind  or  water, 


EXPLOSIVE  FRUITS;    WINGED  FRUITS 


439 


and  if  the  ground  slopes  they  may  easily  roll  far  enough  to  get 
started  in  new  places. 

413.  Dispersal  of  seeds.    Seeds  are  not  infrequently  scattered 
by  apparatus  with  the  aid  of  which  the  plant  throws  them  about. 
More  commonly,  however,  they  depend  upon  other  agencies, 
such  as  wind,  water,  or  animals,  to  carry  them.    Sometimes  the 
transportation  of  seeds  is  due  to  the  structure  of  the  seeds  them- 
selves, sometimes  to  that  of  the  fruit  in  which  they  are  inclosed  ; 
the  essential  point  is  to  have  transportation  to  a  long  distance 
made  as  certain  as  possible,  to  avoid  overcrowding. 

414.  Explosive  fruits.    Some  dry  fruits  burst  open  when 
ripe  in  such  a  way  as  to  throw  their  seeds  violently  about. 
Interesting   studies  may 

be  made,  in  the  proper 
season,  of  the  fruits  of 
the  common  blue  violet, 
the  pansy,  the  wild 
balsam,  the  garden  bal- 
sam, the  cranesbill,  the 
herb  Robert,  the  witch- 
hazel,  the  Jersey  tea,  and 
some  other  common 
plants.  The  capsule  of 
the  tropical  American 
sand-box  tree  bursts  open 
when  throughly  dry  with  FlG>  339>  Fruits  of  linden,  with  a  bract  joined 


a  noise  like  that  of  a  pis- 


to  the  peduncle  and  forming  a  wing 


tol  shot.  The  explosive  force  of  fruits  is  derived  from  the  fact 
that  some  of  their  parts  on  drying  are  left  in  a  state  of  ten- 
sion, some  layers  of  cells  being  compressed  or  stretched  and 
tending  to  readjust  their  position. 

415.  Winged  or  tufted  fruits  and  seeds.  The  fruits  of  the 
ash,  box  elder,  elm,  maple  (Fig.  160),  and  many  other  trees  are 
provided  with  an  expanded  membranous  wing.  Some  seeds,  as 
those  of  the  catalpa  and  the  trumpet  creeper,  are  similarly 


440 


HOW  PLANTS  ARE   SCATTERED 


appendaged.  Winged  fruits  and  seeds  are  borne  on  trees  or 
shrubs,  and  the  wing  is  usually  so  adjusted  as  to  make  its 
descent  slow,  with  a  spinning  motion.  As  a  rule,  winged  fruits 
and  seeds  are  much  heavier  than  those  with  a  tuft  of  hairs. 
The  fruits  of  the  dandelion,  the  thistle,  the  fleabane,  the  arnica 
(Fig.  166,  III),  and  many  other  plants  of  the  group  Compositce, 
to  which  these  belong,  and  the  seeds  of  the  willow,  the  milk- 
weed, the  willow-herb,  and  other 
plants,  bear  a  tuft  of  hairs.  All 
these  seeds  and  fruits  may  in 
windy  weather  be  seen  traveling 
often  to  great  distances. 

416.  Tumble  weeds.  Late  in 
the  autumn,  fences,  particularly 
on  prairie  farms  that  are  not 
carefully  tilled,  or  in  pastures, 
often  serve  as  lodging  places 
for  immense  numbers  of  certain 
dried-up  plants  known  as  tuni- 
bleweeds.  These  blow  about 
over  the  level  surface  until  the 
first  snow  falls  and  even  after 
that  (Fig.  341),  often  traveling 
for  many  miles  before  they  come 
to  a  stop,  and  rattling  out  seeds 
as  they  go.  Some  of  the  com- 
monest tumbleweeds  are  the 
Eussian  thistle  (Salsola  Kali 
var.  Tragus,  Fig.  340),  the  pigweed  (Amarantus  albus,  Fig. 
341),  the  tickle  grass  (Fig.  342),  and  a  familiar  peppergrass 
(Lepidium).  In  order  to  make  a  successful  tumbleweed,  a 
plant  must  be  pretty  nearly  globular  in  form  when  fully  grown 
and  dried,  must  be  tough  and  light,  must  break  off  near  the 
ground,  and  drop  its  seeds  only  a  few  at  a  time  as  it  travels. 
A  single  plant  of  Russian  thistle  is  sometimes  as  much  as 


FIG.  340.   Russian  thistle 
After  Dewey 


DISPERSAL  BY  SHAKING  AND  BY  WATER 


441 


three  feet  high  and  six  feet  in  diameter,  and  carries  not  less 
than  200,000  seeds. 

417.  Many-seeded  pods  with  small  openings.  There  are 
many  fruits  which  act  somewhat  like  pepper  boxes.  The  cap- 
sule of  the  poppy  is  a  good  instance  of  this  kind,  and  the  fruit  of 
lily,  monkshood  (Fig.  159),  columbine,  larkspur,  velvet  leaf  (Abu- 
tilonAvicennce),  and  jimson  weed  (Fig.  343,  C)  acts  in  much  the 
same  way.  Clamping  the  dry  peduncle  of  any  one  of  these  ripe 


FIG.  341.   Tumbleweeds 1  lodged  against  a  wire  fence  in  winter 
After  Millspaugh 

fruits,  so  as  to  hold  it  upright  above  the  table  top,  and  swinging 
it  back  and  forth,  will  readily  show  its  efficiency  in  seed  dispersal. 
418.  Study  of  transportation  by  water.  Nothing  less  than  a 
long  series  of  observations  by  the  pond  margin  and  the  brook- 
side  will  suffice  to  show  how  general  and  important  is  the  work 
done  by  water  in  carrying  the  seeds  of  aquatics.  Many  plants 
usually  have  their  seeds  transported  by  water,  and  some  appear 
to  have  no  provision  for  dissemination  in  any  other  way. 

^Amarantus  albus. 


442 


HOW  PLANTS   ARE  SCATTERED 


Ocean  currents  furnish  transportation  for  the  longest  journeys 
that  are  made  by  floating  seeds.  It  is  a  well-known  fact  that 
cocoa  palms  are  among  the  first  plants  to  spring  up  on  newly 
formed  coral  islands.  The  nuts  from  which  these  palms  grew 
may  readily  have  floated  a  thousand  miles  or  more  without 

injury.  On  examining  a  cocoa- 
nut  with  the  fibrous  husk  at- 
tached, just  as  it  falls  from  the 
tree,  it  is  easy  to  see  how  well 
this  fruit  is  adapted  for  trans- 
portation by  water.  There  are 
altogether  about  a  hundred 
drifting  fruits  known,  one  (the 
Maldive  nut)  reaching  a  weight 
of  twenty  to  twenty-five 
pounds. 

419.  Burs.  A  large  class  of 
fruits  is  characterized  by  the 
presence  of  hooks  on  the  outer 
surface.  These  are  sometimes 
outgrowths  from  the  ovary,  or 
the  style  (as  in  avens),  some- 
times from  the  calyx,  some- 
times from  an  involucre.  Their 
office  is  to  attach  the  fruit  to 
the  hair  or  fur  of  passing  ani- 
mals. Often,  as  in  sticktights 
(Fig.  344  A,  B),  the  hooks  are 
comparatively  weak,  but  in 
other  cases,  as  in  the  cocklebur  (Fig.  344  D),  and  still  more  in 
the  Martynia  (the  fruit  of  which  in  the  green  condition  is  much 
used  for  pickles),  the  hooks  are  exceedingly  strong.  Cockleburs 
can  hardly  be  removed  from  the  tails  of  horses  and  cattle,  into 
which  they  have  become  matted,  without  cutting  out  all  the 
hairs  to  which  they  are  fastened. 


FIG.  342.   Panicle  of  tickle  grass,  a 
common  tumbleweed 

After  Host 


BURS 


443 


A  curious  case  of  distribution  of  this  kind  occurred  in  the 
island  of  Ternate,  in  the  Malay  Archipelago.    A  buffalo  with  his 


FIG.  343.  Three  fruits  adapted  for  dispersal  by  the  shaking  action  of 

the  wind 

A,  celandine;  1>,  pea;  C,  jimson  weed  (Datura).  — After  Decaisne 


FIG.  344.   Burs 

A,  sticktights;  B,  sticktights,  two  segments  (magnified);  (7,  burdock; 
D,  co'ckleburs.  —  After  Kerner 


444 


HOW  PLANTS  ARE  SCATTERED 


hair  stuck  full  of  the  needle-like  fruits  of  a  grass l  was  sent  as 
a  present  to  the  so-called  king  of  Ternate.  Scattered  from  the 
hair  of  this  single  animal,  the  grass  soon  spread  over  the  whole 
island. 

420.  Uses  of  stone  fruits  and  fleshy  fruits  to  the  plant. 
Besides  the  dry  fruits,  of  which  some  of  the  principal  kinds 
have  been  mentioned,  there  are  many  kinds  of  stone  fruits  and 


FIG.  345.   Barbs  and  hooks  of  burs 

A,  barbed  points  from  fruit  of  beggar's  ticks,  x  11 ;  B,  hook  of  cocklebur,  x  11; 
C,  beggar's  ticks  fruit  (natural  size) ;  I),  cocklebur  hook  (natural  size) 

other  fleshy  fruits.  Of  these  the  great  majority  are  eatable  by 
man  or  some  of  the  lower  animals,  and  oftentimes  the  amount 
of  sugar  and  other  food  material  which  they  contain  is  very 
considerable.  It  is  a  well-recognized  principle  of  botany  that 
plants  do  not  make  unrewarded  outlays  for  the  benefit  of  other 
species.  Evidently  the  pulp  of  fruits  is  not  to  be  consumed  or 

lAndropogon  acicularis. 


SEED  CARRYING  DONE  BY  ANIMALS 


445 


used  as  food  by  the  plant  itself  or,  in  general,  by  its  seeds. 
There  are,  therefore,  several  points  to  be  explained  on  the  basis 
of  possible  advantages  to  the  plant.  These  are: 

1.  The  eatable  nature  of  the  pulp  of  many  fruits. 

2.  The  bitter  or  other  unpleasant  taste  of  many  seeds,  as 
those  of  the  orange  and  lemon. 

3.  The  hardness  or  toughness  of  many  seeds  of  pulpy  fruits, 
as  the  date  and  the  peach. 

4.  The  small  size  and  indigestibility  of  seeds  of  pulpy  fruits, 
as  the  fig  and  the  raspberry. 

A  little  observation  in  the  field  suffices  to  show  that  most 
pulpy  fruits  are  habitually  eaten  by  birds  or  other  animals  large 
enough  to  carry  them  away  from  the  parent  plant.  Seeds  of 
disagreeable  flavor,  and  very  large 
hard  seeds  are  often  avoided  by  the 
animal  in  eating  the  fruit  which 
contains  them.  Small  hard  seeds 
are  commonly  swallowed  whole  and 
frequently  remain  nearly  unacted 
upon  by  the  digestive  fluids,  so  that 
they  traverse  the  digestive  tract  of 
the  fruit-eating  animal  which  swal- 
lowed them  and  remain  perfectly 
capable  of  germination.  In  this  way 
such  instances  of  dissemination  as 
those  of  the  raspberry  (Fig.  346)  and 
the  red  cedars  (Fig.  347)  are  readily 
explained. 

421.  Seed  carrying  purposely 
done  by  animals.  In  the  cases  re- 
ferred to  in  the  preceding  sections,  animals  have  been  seen  to 
act  as  unconscious  or  even  unwilling  seed  carriers.  Sometimes, 
however,  they  carry  off  seeds  with  the  plan  of  storing  them  for 
food.  Ants  drag  away  with  them  to  their  nests  certain  seeds 
which  have  fleshy  growths  on  their  outer  surfaces.  Afterwards 


C  FIG.  346.  Red  rasp- 
berry bush,  in  fork 
of  a  maple 


446 


HOW  PLANTS  ARE   SCATTERED 


they  eat  these  fleshy  parts  at  their  leisure,  leaving  the  seed  per- 
fectly fit  to  grow,  as  it  often  does. 


FIG.  347.  Red  cedar  trees  planted  by  birds  roosting  on  fences 
After  Pinchot 

Squirrels  and  blue  jays  are  known  to  carry  nuts  and  acorns 
about  and  bury  them  for  future  use.  These  deposits  are  often 
forgotten  and  so  get  a  chance  to  grow,  and  in  this  way  a  good 
deal  of  tree  planting  is  done. 


FIG.  348.    Seed  of  bloodroot  with  caruncle,  or  crest,  which  serves  as  a  handle 
for  ants  to  hold  on  to.     Ant  ready  to  take  the  seed 

After  Beal 
1  See  Beal,  Seed  Dispersal,  pp.  69,  70. 


CHAPTEE  XXXIV 
SOCIAL  HABITS  OF  PLANTS;   COMPETITION  AND  INVASION 

422.  Social  habits.    Those  plants  which  live  associated  with 
many  individuals  of  the  same  species  are  called  social  plants. 
Those  kinds  which  are  not  social  usually  occur  as  members  of 
plant  communities,  or  assemblages  of  two  or  more  species.    The 
vegetation  of  the  earth  mainly  consists  of  such  assemblages,  and 
the  total  number  of  solitary  plants  is  comparatively  small. 

Adult  seed  plants  are  usually  incapable  of  locomotion,  and 
only  a  small  proportion  of  all  the  kinds  of  seeds  (though  a  some- 
what larger  proportion  of  fruits)  is  equipped  with  means  for 
carrying  them  on  long  journeys.  It  is  therefore  natural  that 
the  offspring  of  any  plant  or  plant  community  should  generally 
be  found  near  the  parent  plants.  It  is  not  easy  to  trace  the 
working  of  this  gradual  spread  of  the  successive  broods  in 
the  neighborhood  of  the  parents  where  there  is  already  dense 
vegetation.  But  in  any  region  where  there  are  considerable  areas 
destitute  of  any  given  vegetation  form,  as  in  cleared  land,  the 
young  seedlings  of  an  oak,  a  hickory,  or  a  black  walnut  may 
often  be  detected  in  many  places  near  the  parent  tree. 

423.  Competition.    Every  one  knows,  in  a  general  way,  that 
in  a  state  of  nature  plants  often  greatly  crowd  each  other.    This 
is    evident  enough  from  mere  inspection  of    most    meadows, 
thickets,  or  tracts  of  woodland  or  waste  land ;  but  in  order  to 
realize  how  few  of  all  the  bidders  for  each  square  foot  of  ground 
actually  find  a  chance  to  occupy  it,  a  little  calculation  is  needed. 
A  single  annual  seed  plant  usually  matures  hundreds  and  often 
thousands  of  seeds.    One  common  weed  of  the  Middle  West,  the 
Russian  thistle1  (Fig.  340),  often  produces  as  many  as  25,000 

1  Sdlsola  Kali  var.  Tragus. 
447 


448  COMPETITION   AND  INVASION 

seeds  and  occupies  as  much  as  four  square  feet  of  earth.  The 
offspring  of  an  individual  of  this  species,  therefore,  if  all  the  seeds 
grew  to  mature  plants,  would  cover  nearly  2.3  acres.  It  may 
interest  the  student  to  calculate  in  what  generation  the  descend- 
ants of  one  plant  would  cover  the  entire  area  of  his  state. 

424.  Statistics  of  overcrowding.  Charles  Darwin  seems  to 
have  been  one  of  the  earliest  observers,  if  not  the  very  first,  to 
collect  exact  statistics  in  regard  to  the  severity  of  competition 
among  plants.  He  found  that  out  of  20  species  which  occurred 
on  a  plot  of  turf  three  by  four  feet  in  area  nine  species  died 
from  overcrowding  by  the  others.  On  a  piece  of  dug  and  cleared 
ground  he  found  that  60  weed  seedlings  to  the  square  foot 
sprang  up  and  49  of  them  were  destroyed,  chiefly  by  slugs 
and  insects.1 

In  a  rich  and  weedy  bit  of  land  Professor  L.  H.  Bailey 
found  in  an  area  of  twenty  by  twenty  square  inches  ten  spe- 
cies of  weeds.  Reduced  to  the  number  per  square  foot,  there 
were:  July  10,  30  plants;  August  13,  31  plants  ;  September  25, 
25  plants.  Several  of  these  were  large  weeds,  such  as  the 
redroot  (Amarantus  retroftexus)  and  the  ragweed  (Ambrosia 
artemisicefolia) . 

On  June  23  of  the  next  year  there  were  on  the  same  plot 
(which  had  remained  undisturbed)  eleven  species,  numbering 
108  plants  to  the  square  foot,  and  now  the  dominant  plants 
were  red  clovers.  Most  of  the  other  plants  were  puny  and 
suffering  from  lack  of  light  under  the  shade  of  the  clovers.2 

If  one  selects  a  plot  hi  which  seedlings  are  just  starting,  the 
number  of  individuals  to  the  square  foot  will  often  be  found  to 
be  much  greater  than  those  above  given.  Under  a  full-grown 
tree  of  the  wild  black  cherry  the  writer  has  found  on  June  9 
portions  of  the  ground  containing  hardly  any  other  seed  plants 
except  cherry  seedlings  at  the  rate  of  104  to  the  square  foot. 
Not  one  of  all  the  thousands  which  had  begun  to  grow  could 

1  Origin  of  Species,  Chapter  in. 

2  The  Survival  of  the  Unlike,  pp.  258-261. 


STATISTICS  OF  OVERCROWDING 


449 


ever  have  developed  into  a  full-sized  individual  on  account  of 
the  overshadowing  from  the  parent  tree, 

In  a  weedy  bit  of  lawn,  where  the  grass  had  largely  been 
killed  by  trampling  and  other  disturbing  causes,  the  writer 
found  on  June  9  plants  at  the  rate  of  1032  to  the  square 
foot  as  follows : 

Plantain  (Plantago  Riiyelii) 811 

Grass  (various  species) 200 

Knotgrass  (JPolygonum  aviculare) 18 

Sorrel  (Oxalis  corniculata  var.  slricta) 3 

1032 


A  B 

FIG.  349.   Effect  of  competition  on  radishes 

Both  plants  were  grown  from  the  same  seed  and  in  the  same  soil,  planted  at  the 
same  time.  A  was  one  of  a  lot  standing  so  close  together  that  their  tap  roots 
nearly  touched  one  another;  B  had  several  square  feet  of  ground  to  itself. 
About  one-quarter  natural  size 


450  COMPETITION  AND  INVASION 

The  majority  of  the  grass  plants  were  apparently  seedlings  of 
the  preceding  autumn,  and  the  plantains  were  young  seedlings, 
most  of  them  an  inch  or  less  in  height.  A  full-grown  plantain 
of  this  species  occupies  not  less  than  100  to  150  square  inches, 
so  that  of  these  alone  more  than  800  individuals  were  likely  to 
die  of  overcrowding. 

425.  How  overcrowding  kills.    Of  plants  grown  too  close 
together  many  die  and  others  are  dwarfed  (Fig.  349)  and  par- 
tially or  wholly  fail  to  flower  or  seed.    This  is  one  of  the  first 
lessons  which  the  beginner  in  gardening  learns,  if  he  neglects 
properly  to  thin  out  his  beds.    Corn  grown  in  closely  planted 
drills  for  fresh  fodder  or  ensilage  makes  few  ears,  and  none  of 
these  are  perfect.    The  weakening  or  destruction  due  to  over- 
crowding results  mainly  from  these  three  causes 1 : 

1.  Insufficient  light  and  heat  for  plants  shaded  by  their  more 
vigorous  neighbors,  resulting  in  imperfect  photosynthesis. 

2.  Scanty  water  supply,  because  most  of  the  water  is  absorbed 
by  the  more  vigorous  root  systems  of  the  stronger  individuals. 

3.  Deficient  supply  of  dissolved  salts  (nitrates,  phosphates, 
and  so   on),  on  account  of  their  being  largely  consumed   by 
the  stronger  plants. 

426.  Competition  most  fatal  between  similar  plants.    For 
obvious  reasons,  plants  of  the  same  general  form  and  mode  of 
growth  usually  interfere  most  with  each  other,  and  plants  which 
are  decidedly  unlike  interfere  less,  or  even  in  some  cases  benefit 
each  other.    This  principle  is  unconsciously  followed,  in  many 
instances,  by  farmers  and  gardeners,  as  in  the  case  of  lawns 
sown  with  mixed  grass  seed,  which  produce  a  more  perfect  turf 
than  those  sown  with  a  single  species  of  grass.    So,  too,  pumpkins 
are  often  planted  in  cornfields,  and  in  southern  Europe  beans 
are  raised  in  vineyards,  in  the  partial  shade  of  the  vines. 

If  the  interests  of  two  or  more  kinds  of  plants  occupying  the 
same  area  conflict  little  or  not  at  all,  this  may  be  due  not  only 
to  their  unlikeness  of  form  or  of  requirements  as  regards  light, 
1  As  far  as  terrestrial  seed  plants  are  concerned. 


CASES  OF   INVASION  451 

heat,  or  water,  but  also  to  their  flowering  and  seeding  at  differ- 
ent seasons.  Many  kinds  of  weeds  nourish  in  grainfields,  mak- 
ing little  growth  until  the  grain  is  reaped,  after  which  they 
develop  rapidly  and  flower  and  seed  among  the  stubble. 

427.  Invasion.    Some  of  the  ways  in  which  plants  are  dis- 
persed have  already  been  described  (Chapter  xxxin).    The  result 
of  carrying  seeds  or  other  reproductive  parts  into  new  territory 
is  to  cause  an  invasion  of  that  area.    If  the  invaded  ground  con- 
tains no  vegetation,  the  newcomers  take  full  possession.    Such  a 
case  occurs  when  the  bed  of  a  newly  drained  lake  or  bayou,  or 
soil  uncovered  by  landslides,  or  newly  cooled  material  from  vol- 
canic eruptions  is  populated  by  vegetation  brought  in  by  natural 
agencies.    If  the  invading  species  encounter  other  occupants  of 
the  region  invaded,  the  new  arrivals  may  simply  share  the  ter- 
ritory with  its  previous  occupants.    But  if  the  immigrants  are 
much  better  adapted  to  the  conditions  of  existence  in  the  dis- 
puted area  than  are  its  actual  occupants,  the  intruders  may  drive 
out  all  before  them. 

428.  Native  species  ousted  by  invaders.    New  Zealand  and 
the  pampas  of  La  Plata  and  Paraguay,  in  South  America,  have, 
during  the  nineteenth  century,  furnished  wonderful  examples  of 
the  spread  of  European  species  of  plants  over  hundreds  of  thou- 
sands of  square  miles  of  territory.    The  newcomers  were  more 
vigorous,  or  in  some  way  better  adapted  to  get  on  in  the  world, 
than  the  native  plants  which  they  encountered,  and  so  managed 
to  crowed  multitudes  of  the  latter  out  of  existence. 

In  our  own  country  a  noteworthy  case  of  the  kind  has 
occurred  so  recently  that  it  is  of  especial  interest  to  Ameri- 
can botanists.  The  so-called  Russian  thistle  (Fig.  340),  which 
is  merely  a  variety  of  the  saltwort  common  along  the  Atlan- 
tic coast,  was  first  introduced  into  South  Dakota  in  flaxseed 
brought  from  Russia  and  planted  in  1873  or  1874.  In  twenty 
years  from  that  time  the  plant  had  become  generally  distributed 
as  one  of  the  commonest  weeds  over  an  area  of  about  25,000 
square  miles. 


452  COMPETITION  AND  INVASION 

American  plants,  on  the  other  hand,  have  in  many  cases 
overrun  other  countries.  -Elodea,  a  common  water  weed  with 
us,  introduced  into  Great  Britain  about  1847,  now  chokes  up 
many  pools  and  water  courses  in  England  and  Scotland.  The 
prickly  pear  cactus  (Opuntia  Ficus-indica)  and  the  century 
plant,  both  emigrants  from  North  America,  are  now  the  most 
conspicuous  plants  along  many  cliff  sides  all  over  southern  Italy. 
A  prickly  pear  has  become  such  a  nuisance  in  New  South  Wales 
that  large  rewards  are  offered  for  its  extermination. 

429.  Weeds.  Any  flowering  plant  which  is  troublesome  to 
the  farmer  or  gardener  is  commonly  known  as  a  weed.  Though 
such  plants  are  annoying  from  their  tendency  to  crowd  out 
others  useful  to  man,  they  are  of  extreme  interest  to  the  bot- 
anist on  account  of  this  very  hardiness.  The  principal  charac- 
teristics of  the  most  successful  weeds  are  their  ability  to  live 
in  a  variety  of  soils  and  exposures,  their  rapid  growth,  resist- 
ance to  frost,  drought,  and  dust,  their  unfitness  for  the  food 
of  most  of  the  larger  animals,  in  many  cases  their  capacity  to 
accomplish  self  pollination,  in  default  of  cross  pollination,  and 
their  ability  to  produce  many  seeds  and  to  secure  their  wide 
dispersal. 

Sometimes  the  seeds  have  great  vitality ;  those  of  shepherd's 
purse  and  purslane  are  capable  of  germinating  after  fifteen 
years  or  more.  Many  of  the  worst  weeds,  such  as  sow  thistle,1 
sorrel,2  witch  grass,3  nut  grass,4  and  field  garlic,5  have  creeping 
rootstocks  or  bulbs  or  tubers.  Not  every  weed  combines  all  of 
these  characteristics.  For  instance,  the  velvet  leaf,  or  butter 
print,  common  in  cornfields,  is  very  easily  destroyed  by  frost; 
the  pigweed  and  purslane  are  greedily  eaten  by  pigs,  and  the 
ragweed  by  some  horses.  The  horse-radish  does  not  usually  pro- 
duce any  seeds,  but  is  propagated  by  vegetative  methods. 

It  is  a  curious  fact  that  many  plants  which  have  finally 
proved  to  be  noxious  weeds  have  been  purposely  introduced 
into  the  country.  The  fuller's  teasel,  melilot,  horse-radish,  wild 

1  Sonchus.         2  Rumex.        3  Agropyrum.        4  Cyperus.        5  Allium. 


ORIGIN  OF  WEEDS  453 

carrot,  wild  parsnip,  tansy,  oxeye  daisy,  and   field  garlic  are 
instances  of  this. 

430.  Origin  of  weeds.1  By  far  the  larger  proportion  of  our 
weeds  are  not  native  to  this  country.  Some  have  been  brought 
from  South  America  and  from  Asia,  but  most  of  the  introduced 
kinds  come  from  Europe.  The  importation  of  various  kinds  of 
grain  and  of  garden  seeds,  mixed  with  seeds  of  European  weeds, 
will  account  for  the  presence  of  many  of  the  latter  among  us. 
Others  have  been  brought  over  in  the  ballast  of  vessels.  Once 
landed,  European  weeds  have  succeeded  in  establishing  them- 
selves in  so  many  cases,  because  they  were  superior  in  vitality 
and  in  their  power  of  reproduction  to  our  native  plants.  This 
may  not  improbably  be  due  to  the  fact  that  the  European  and 
western  Asiatic  vegetation,  much  of  it  consisting  from  very  early 
times  of  plants  growing  in  comparatively  treeless  plains,  has  for 
ages  been  habituated  to  flourish  in  cultivated  ground  and  to 
contend  with  the  crops  which  are  tilled  there. 

1  See  the  article,  "Pertinacity  and  Predominance  of  Weeds,"  in  Scientific 
Papers  of  Asa  Gray,  selected  by  C.  S.  Sargent,  Vol.  II,  pp.  234-242. 


CHAPTEE  XXXV 
PLANT  SUCCESSIONS* 

431.  Nature  of  plant  successions.    Whenever  a  portion   of 
the  earth's  surface  is  stripped  of  its  vegetation,  or  undergoes 
any  decided  change  in  its  physical  condition,  the  way  is  usually 
opened  for  invasion  of  plants  from  the  surrounding  territory 
(Sec.  427).    In  most  cases  the  immigrants  are  not  all  of  them 
thoroughly  adapted  to  their  new  home,  and  cannot  become  so ; 
or  the  condition  of  the  territory  may  continue  to  change,  so 
that  a  series  of  new  populations  appears,  each  in  turn  wholly 
or  partly  giving  way  to  that  which  follows  it.    Such  a  set  of 
colonizings  is  called  a  plant  succession. 

432.  Causes   of   successions.    It    would    require    too    much 
space   to    state  more  than   a  very   few   of  the   causes  which 
originate  plant  successions. 

First.  They  may  be  brought  about  by  the  introduction  into 
a  region  of  new  species  which  are  able,  without  change  of  soil 
or  climate,  to  drive  out  some  or  all  of  the  original  occupants 
(Sec.  428). 

Second.  They  may  be  brought  about  by  changing  the  supply 
of  light,  heat,  water,  or  other  important  factors  in  the  surround- 
ings of  the  plant.  Such  changes  are  sometimes  natural,  some- 
times produced  by  man. 

Such  a  river  as  the  Mississippi,  with  over  12,000  square 
miles  in  its  delta,  affords  a  good  instance  of  the  power  of  natural 
agencies  to  alter  the  conditions  of  plant  life.  Perhaps  one  third 
of  the  delta  is  a  sea  marsh,  with  the  vegetation  characteristic  of 

*  To  THE  INSTRUCTOR  :  As  this  chapter  is  somewhat  more  technical  than 
many  of  the  others  of  Part  III,  it  may  be  omitted  if  limitations  of  time 
demand  a  briefer  course. 

454 


CAUSES  OF  SUCCESSIONS  455 

shallow,  salt,  or  brackish  water  in  a  warm-temperate  climate, 
while  the  remaining  portion  supports  in  places  a  most  luxuriant 
growth  of  land  plants.  Year  by  year,  along  the  margin  of  the 


FIG.  350.   Aspen  succession  after  forest  fires  in  coniferous  woods,  Colorado 

After  Clements 

submerged  part  of  the  delta,  as  this  emerges  from  the  water, 
the  change  from  aquatic  to  land  vegetation  goes  on,  and  year  by 
year  the  flora  which  first,establishes  itself  in  the  newly  emerged 
mud  is  succeeded  by  others  more  adapted  to  ordinary  soil. 


456  PLANT   SUCCESSIONS 

Man  produces  most  extensive  changes  in  vegetation  by  such 
operations  as  draining  lakes  and  swamps,  building  levees,  irrigat- 
ing deserts  and  semi-deserts,  clearing  woodlands,  and  planting 
treeless  lands  with  the  seeds  of  forest  trees. 

433.  Order  of  succession  in  special  cases.  Much  study  has 
recently  been  given  to  the  exact  order  in  which  assemblages  of 
plants  follow  each  other  in  various  kinds  of  succession.  Only 
a  very  few  cases  can  here  be  mentioned. 

On  the  island  of  Krakatoa,  which  was  completely  laid  waste 
by  a  volcanic  eruption  in  1883,  the  first  forms  of  plant  life  to 
appear  were  microscopic  blue-green  algae  (Sees.  207—211).  Three 
years  after  the  eruption  the  flora  had  come  to  contain  many 
ferns,  with  here  and  there  a  few  seed  plants,  on  the  mountains 
or  the  coast. 

In  the  mountains  of  Colorado  the  granite  bowlders  dislodged 
from  the  faces  of  cliffs  are  covered  first  with  incrusting  lichens ; 
then  the  gravel  produced  by  the  weathering  of  the  granite  gives 
a  footing  to  leaf -like  lichens ;  later  the  more  weathered  gravel 
supports  a  growth  of  herbaceous  seed  plants ;  afterward  follow 
thickets,  then  pine  forests,  and  finally  spruce  forests  (Plate XII). 

In  the  pine  woods 1  of  central  Maine  when  the  trees  have 
been  cut  away  and  the  clearing  (as  is  too  often  the  case)  burned 
over,  the  most  conspicuous  plants  which  immediately  succeed 
the  forest  are  fireweed,2  raspberries,  blackberries,  wild  cherries,3 
and  aspens.4  A  deciduous  forest  of  poplars  and  canoe  birches  5 
succeeds  the  thickets  above-mentioned.  This  in  turn  would 
doubtless,  under  natural  conditions,  after  a  long  period,  be  dis- 
placed by  a  pine  forest. 

In  eastern  Maine  the  succession  is  very  similar,  except  that 
blackberries  are  not  common  in  the  burned  clearings  and  the 
tree  growth  which  follows  the  thicket  stage  is  usually  of  gray 
birch.6 

1  Pinus  Strjbus.  4  Populus  tremuloides. 

2  EpiloUum  angustifolium.  5  Betula  papyrifera. 

3  Prunus  pennsylvanica.  6  B.  populifolia. 


REASONS  FOR  ORDER  OF  SUCCESSION 


457 


434.  Reasons  for  order  of  succession.  It  is  not  always  pos- 
sible to  explain  in  detail  why  each  set  of  plants  in  a  succession 
takes  possession  of  the  ground  and  later  on  is  itself  driven  out. 


FIG.  351.  Young  black  oaks  succeeding  loblolly  pine  and  shortleaf  pine, 

southeastern  Texas 

After  von  Schrenk 

In  a  general  way  it  is  clear  that  very  low  spore  plants  can 
make  a  living  on  bare  rock  surfaces  or  on  partly  decomposed 
rock  where  seed  plants  would  find  too  little  available  salts, 
especially  nitrates,  to  support  their  nutrition. 


458  PLANT   SUCCESSIONS 

On  lands  where  cultivation  is  abandoned,  or  which  are  in 
other  ways  suddenly  exposed  to  invasion,  weeds  of  many  spe- 
cies often  obtain  a  footing  and  nourish  for  some  years  before  the 
truly  wild  native  plants  of  the  region  take  final  possession.  This 
is,  in  part  at  least,  on  account  of  the  remarkable  capacity  of  most 
weeds  to  seed  themselves  (Sec.  429). 

Forest  or  grass  land  is  the  final  stage  in  many  successions. 
The  former  gains  supremacy  over  the  weedy  thickets  out  of 
which  it  rises  by  shading  the  shrubs  and  herbs  beneath  the 
tree  tops  until  all  those  not  adapted  to  life  in  deep  shade  are 
destroyed.  Grasses  have  to  an  unsurpassed  extent  the  power  of 
living  with  their  roots  (and  sometimes  also  rootstocks)  inter- 
woven in  a  way  which  would  prove  fatal  to  most  herbs.  In 
this  way  a  lawn  or  meadow,  on  good  ground,  may  be  seen  to 
improve  itself  by  choking  out  other  plants  which  occur  here 
and  there  among  the  grass.  Salt  marshes,  with  a  comparatively 
scanty  vegetation,  are  often  purposely  shut  away  from  the  sea, 
so  that  the  rains  can  wash  the  excess  of  salt  out  of  the  soil.  In 
four  or  five  years  they  become  thoroughly  self-sown  with  the 
seeds  of  cultivated  grasses  and  are  changed  into  highly  produc- 
tive meadows. 


CHAPTEE  XXXVI 
ECOLOGICAL  GROUPS  AND  THEIR  CHARACTERISTICS  * 

435.  Ecological  grouping  of  plants.  The  ordinary  classifi- 
cation of  plants,  as  set  forth  in  Part  II,  is  based,  as  far  as 
possible,  on  their  actual  relationships  to  each  other.  But  when 
plants  are  considered  ecologically  they  are  grouped  according  to 
their  relations  to  the  world  about  them.  They  may,  therefore, 
be  gathered  into  as  many  (or  more  than  as  many)  different 
groups  as  there  are  important  factors  influencing  their  modes  of 
life.  We  may,  for  instance,  classify  plants  as  light-loving  and 
shade-loving,  and  so  on. 

The  most  important  consideration  in  classifying  seed  plants 
on  ecological  grounds  is  based  on  their  requirements  in  regard 
to  water.  Grouped  with  reference  to  this  factor  in  their  life  all 
plants  may  be  designated  as  : 

1.  Hydrophytes,  or  water-inhabiting  or  water -tolerating  plants. 

2.  Xerophytes,  or  drought-tolerating  plants. 

3.  Mesophytes,  or  plants  which  thrive  best  with  a  moderate  supply 

of  water. 

These  three  groups  do  not  fully  express  all  the  relations  of  plants 
to  the  water  supply,  so  two  others  are  found  convenient : 

4.  Tropophytes,  or  seasonal  plants  which  are  hydrophytes  during  part 

of  the  year  and  xerophytes  during  another  part.1 

5.  Halophytes,   or  salt-marsh  plants  and   "  alkali  "  plants,   species 

which  can  flourish  in  a  very  saline  soil. 

*  To  THE  INSTRUCTOR  :  If  it  is  necessary  to  shorten  the  treatment  of  this 
subject,  the  latter  part  of  the  chapter,  beginning  with  Sec.  442,  may  be 
omitted, 

1  The  plants  which  E.  Warming,  one  of  the  foremost  authorities,  classes 
as  mesophytes  are  many  of  them  grouped  by  another  great  authority, 
A.  F.  W.  Schirnper,  as  tropophytes. 

459 


460 


ECOLOGICAL  GROUPS 


436.  Difficulties  in  ecological  grouping.  It  seems  at  first  sight 
a  simple  matter  to  group  plants  in  regard  to  their  need  of 
water.  There  can  be  no  difficulty  in  recognizing  as  hydrophytes 
all  plants  like  the  bladderworts,  water  cresses,  certain  mosses, 
and  most  algse  which  live  only  in  water.  Cactuses,  aloes,  and 
similar  plants  are  recognized  at  sight  as  xerophytes.  But  the 
chief  difficulty  is  in  dividing  mesophytes  from  the  other  two 
assemblages,  into  which  they  shade  by  indefinite  gradations.  In 
a  single  mesophytic  thicket,  for  example, 
one  may  find  such  hydrophytes  as  the 
pepper  bush  (Cletlirn)  and  such  moderate 


FIG.  352.  Aquatic  plants  :  pond  lilies  with  floating  leaves, 
and  sedges  with  aerial  leaves 

xerophytes  as  the  catbriers  (Smilax).  In  order  to  know  whether 
the  plants  of  a  region  have  plenty  of  water  or  not,  we  must  know 
not  only  how  many  inches,  of  yearly  rainfall  there  are,  but  also 
what  the  soil  is  like,  what  is  the  temperature  of  the  soil  and  air? 
whether  or  not  there  are  dry  winds,  and  whether  there  are  fogs  or 
heavy  dews.  A  lichen  on  a  bare  rock  may  be  living  almost  under 
desert  conditions,  while  a  pitcher  plant  in  a  bog  near  by  has  its 
roots  in  standing  water  (or  in  ice)  nearly  all  the  year  round. 


HYDROPHYTES 


461 


437.  Hydrophytes.  Some  of  these  are  herbaceous  aquatic 
plants,  like  the  duckweed,  the  pickerel  weed,  the  pond  lily, 
and  the  water  crowfoot ;  others,  such  as  the  cultivated  "  calla  " 
(JKichardia) ,  the  buck  bean,  the  cat-tail,  and  the  sweet  flag,  many 
ferns,  mosses,  and  liverworts,  prefer  damp  air  and  soil.  All  of 
them  transpire  freely,  and  many  of  them  cannot  live  at  all 
under  the  moisture  conditions  which  suit  ordinary  plants. 


FIG.  353.  Submerged  and  aerial 
leaves  of  a  crowfoot  (Ranunculus 
aquatilis) 

The  leaf  with  thread-like  divisions  is 
the  submerged  one.  —  After  Giesen- 
hagen 


FIG.  354.  Cross  sections  of 
leaves  of  arrowhead  (Sag- 
ittaria) 

A,  aerial  leaf;  B,  submerged 
leaf.  The  submerged  leaf 
has  no  ordinary  epidermis 
and  no  palisade  layer,  but 
large  air  spaces.  Much 
magnified.  —  After  Bonnier 
and  Sablon 


Some  aquatics  have  their  leaves  wholly  submerged,  others, 
such  as  the  duckweed  (Fig.  355)  and  the  pond  lilies  (Fig.  352), 
have  them  floating,  and  still  others,  like  the  sedges  in  the  same 
illustration,  have  their  leaves  freely  exposed  to  the  air.  A  few 
plants  have  both  water  leaves  and  air  leaves  (Fig.  353).  Some 
aquatic  plants  are  rooted  in  the  mud,  while  others  have  no  roots 
at  all,  or,  like  the  duckweed,  have  only  water  roots.1 
1  See  grouping  in  Sec.  454. 


462 


ECOLOGICAL  GROUPS 


The  leaves  of  land  plants  in  very  rainy  sub-tropical  climates 
are  exposed  to  the  attacks  of  parasitic  fungi.    To  ward  off  the 

-. .  ..,-._„_  attacks  of  these  and  to 

allow  free  transpiration, 
it  is  necessary  to  keep 
water  from  accumulating 
on  the  surfaces  of  the 
leaves.  This  result  is 
secured  by  a  waxy  de- 
posit on  the  epidermis, 
and  also  by  the  slender 


FIG.  355.   The  duckweed,  a  floating 
aquatic  plant 

prolongation  to  drain  off  surplus  water, 
shown  in  Fig.  356.  That  this  slender  leaf 
tip  is  useful  in  the  way  suggested  is  proved 
by  the  fact  that  when  it  is  cut  squarely  off 
the  leaf  no  longer  sheds  water  freely. 

438.  Xerophytes.  A  xeropliyte  is  a  plant 
which  is  capable  of  sustaining  life  with  a 
very  scanty  supply  of  water.  Since  the  first 
plants  which  existed  were  aquatics,  we 
must  consider  that  xerophytes  are  highly 
specialized  and  modified  forms  adapted  to  East  Indian  fig  tree1 
extremely  trying  conditions  of  life.  A  typi- 
cal xerophyte  is  one  which  can  live  in  a 
very  dry  soil  in  a  nearly  rainless  region. 
The  yucca  and  the  cactuses  (Figs.  50,  357) 
are  good  examples  of  such  plants.  Less  extremely  xerophytic 
are  plants  like  the  date  palm  (Fig.  53),  which  flourishes  in  the 

1  Ficus  religiosa. 


with  a  slender,  taper- 
ing point  to  drain  off 
water 

After  Schimper 


PLATE  IX. 

The  upper  picture  shows  a  belt  of  trees  along  a  Nebraska  river 
After  U.  G.  Cornell 

The  lower  picture  shows  xerophytic  grasses  on  Nebraska  sand  hills 
After  R.  A.  Emerson 


FIG.  357.   A  field  of  prickly  pear  cactus  in  California 


463 


464 


ECOLOGICAL   GROUPS 


oases  of  the  Sahara,  where  the  soil  is  moist  from  the  presence 
of  springs,  though  rains  are  almost  unknown,  or  the  houseleeks 
and  stonecrops  found  in  many  gardens,  the  so-called  Spanish 
moss  (Plate  III),  and  lichens  (Figs.  226,  227),  all  of  which  grow 
most  rapidly  in  moist  air,  but  cling  to  bare  rocks  and  trunks 
of  trees,  from  which  they  get  no  water. 

It  is  important  to  notice  that  many  xero- 
phytes only  economize  water  ivhen  forced  to 
do  so.  With  an  abundant  supply  of  water 
they  may  transpire  almost  or  quite  as  much 
as  mesophytes.  But  a  drought  which 
would  kill  the  latter  would  only  cause  the 
xerophytes  to  close  their  stomata  and  greatly 
lessen  transpiration.  A  xerophyte  must  be 
capable  of  storing  water  and  transpiring 
very  slowly,  like  cactuses,  aloes,  stonecrops, 
and  such  fleshy  plants  with  a  thick  epider- 
mis, or  else  it  must  be  able  to  revive  after 
being  thoroughly  dried. 

439.  Roots  and  stems  of  xerophytic  seed 
plants.  Some  xerophytes  have  roots  which 
show  no  peculiarities  of  form  or  structure, 
but  many  make  special  provision  for  storing- 
food  and  water  in  their  roots.  Such  roots 

FIG.  358.   Ilarpago-   are  fleshy  and  often'  as  in"  Harpagopliytnm 
phytum,  a  South   (Fig.  358),  are  of  great  size  compared  with 
African  xerophyte     the  pOrtion  of  the  plant  above  the  ground. 
After  Schimper         Xerophytic  stems  are  frequently  very  thick 
in  proportion  to  their  length,  sometimes  even  globular  (Fig.  50), 
and  they  commonly  contain  large  amounts  of  water.     In  leaf- 
less plants,  like  the  cactuses,  the  surface  for  transpiration  is 
much  less   than  that  offered  by  leafy  plants.    Many  species 
which  bear  leaves  shed  most  of  them  at  the  beginning  of  the 
dry  season,  and  some  remain  thus  in  a  half  dormant  condition 
for  long  periods,  as  is  the  case  with  many  Euphorbias  (Fig.  318). 


LEAVES  OF  XEROPHYTES 


465 


The  epidermis,  even  on  the  younger  portions  of  the  stem,  is 
highly  cutinized,  and  this  structure  makes  any  evaporation 
very  slow. 

440.  Leaves  of  xerophytes.  Since  the  leaf  is  hi  general  the 
main  organ  of  transpiration,  we  might  expect  to  find  the  leaves 
of  xerophytes  highly  adapted  to  their  environment.  This  is  the 


FIG.  359  FIG.  360 

FIG.  359.    Cross  section  of  leaf  of  Ficus  elastica 

• 

c,  cuticle ;  o,  opening  to  pit ;  p,  pit  leading  to  stoma ;  s,  stoma ;  e,  epidermal  cells ; 
w,  special  cells  for  storage  of  water;  ch,  air  chamber  from  stoma;  sp,  cells 
of  spongy  parenchyma ;  a,  intercellular  air  spaces.  Much  magnified 

FIG.  360.   Fleshy  leaves  of  Mesembryanthenmm,  with  stored  water 
After  Giesenhagen 

case,  and  some   of  their   principal   means  of  protection   from 
excessive  transpiration  are  as  follows  l : 

1.  A  thick  epidermis,  often  of  several  layers  of  cells  (Fig.  359). 

2.  Storage  of  water  in  epidermal  cells. 

3.  Small  stomata,  often  deeply  sunken  (Fig.  359). 

4.  Epidermal  hairs  or  scales.    These  are  often  extraordinarily 
abundant,  and  in  some  cases  give  one  or  both  surfaces  of  the 
leaf  a  silky  or  silvery  luster. 

1  See  Warming,  Lehrbuch.  der  (Ekologischen  Pflanzengeographie,  vierter 
Abschnitt,  Berlin,  1902. 


466 


ECOLOGICAL   GROUPS 


5.  Coatings  of  wax  or  varnish  or  incrustations  of  salts. 

6.  Extreme  development  of  the  palisade  layer. 

7.  Eeduction  of  the  intercellular  spaces. 

8.  Mucilaginous,  water-retaining  cell  contents  in  the  spongy 

parenchyma  of  the 
leaf  (usually  in  fleshy 
leaves,  Fig.  360). 

9.  Permanent  verti- 
cal position  of  leaves 
(Figs.  45,  110,  111). 

10.  Leaf    move- 
ments, presenting  only 

FIG.  361.   Cross  section  of  rolled-up  leaf  of 
crowberry  (Empetrum  nigrum] 

Magnified.  —  After  Kerner 

11.  Rolling  up  of  leaves,  either  permanent,  as  in  Fig.  361, 
or  temporary,  as  in  Indian  corn  and  in  Fig.  362. 

12.  Eeduction  of  leaf  area,  —  the  leaves  either  few  or  small, 
or  both.    Sometimes  the  leaf  consists  of  little  else  besides  a 
petiole  ;  sometimes,  as  in  Figs.  50  and  357, 

foliage  leaves  are  wholly  absent. 


the  edges  to  the  sun 
during  the  heat  of  the 
day  (Sec.  114). 


FIG.  362.    Cross  section  of  leaves  of  a  grass,1  unrolled  for  exposure  to  sun- 
light and  rolled  up  to  prevent  evaporation 

r,  ridges  of  the  upper  epidermis,  with  many  stomata  on  their  surfaces ;  e,  thick 
lower  epidermis,  without  stomata.  —  After  Kerner 

In  regions  with  a  long  rainless  summer,  like  that  of  southern 
California  or  the  coast  of  the  Mediterranean,  many  shrubs  are 
summer  deciduous,  and  in  their  leafless  condition  the  twigs 


Stipa  capillata. 


MESOPHYTES 


467 


have  been  found  to  transpire  less  than   3   per  cent  of  their 
maximum  rate  when  leafy. 

Some  of  the  principal  differences  between  hydrophytes  and 
xerophytes  may  be  summed  up  as  follows : 


HYDROPHYTES 

XKROPHYTKS 

Roots                 

Few 

Many 

Water-conducting  tissue    . 
Air-conducting  tissue    .... 
Water-storage  tissue     .... 
Epidermis     

Scanty 
Abundant 
Wanting 
Thin  or  wanting 
Often  large  or  dis- 

Abundant 
Scanty 
Often  abundant 
Thick 
Usually  of  reduced 

sected 

surface 

441.  Mesophytes.    A  mesophyte  is  a  plant  which    thrives 
best  with  a  moderate  supply  of  water.    The  great  majority  of 
the  wild  and  the  cultivated  plants  of  the  United  States  are 
mesophytes.    What  has  been  learned  from  Part  I  of  this  book 
about  the  forms,  structure,  and  habits  of  ordinary  plants,  to- 
gether with  what  the  student's  own  observation,  aside  from  the 
study  of  botany,  has  taught  him,  should  suffice  to  give  him  a 
fair  idea  of  mesophytic  plant  life. 

It  is  important  to  notice  that  most  of  our  mesophytic  trees 
and  shrubs  pass  the  winter  (or  in  the  extreme  Southwest  the 
dry  season)  in  a  leafless  condition,  and  so  transpire  very  little. 
So,  too,  our  mesophytic,  herbaceous  perennials,  such  plants  as 
the  jack-in-the-pulpit,  lilies,  irises  (Fig.  45),  violets,  and  others, 
lose  a  large  portion  of  their  evaporating  surface  during  part  of 
the  year  by  dying  to  the  ground  and  leaving  only  the  buried 
bulbs,  roots  with  buds  at  the  crown,  or  rootstocks  alive. 

All  of  the  plants  which  make  decided  preparations  for  the 
season  when  water  is  hard  to  get  may  be  classed  as  tropophytes 
or  periodic  xerophytes. 

442.  Deciduousness  an  acquired  habit.    The  practice  of  shed- 
ding the  leaves  before  the  arrival  of  severe  freezing  weather, 
when  it  becomes  almost  impossible  to  draw  moisture  from  the 


468 


ECOLOGICAL   GROUPS 


earth,  or  before  the  culmination  of  the  severest  drought  of  sum- 
mer, may  be  regarded  as  a  habit  gradually  acquired  by  decidu- 
ous trees  and  shrubs  for  their  own  protection.  The  duration  of 
the  period  of  leaflessness  depends  on  the  length  of  the  danger- 
ous season.  Grapevines,  for  instance,  in  central  Europe  are  leafy 
during  about  six  months  and  leafless  during  the  following  six. 
But  near  Cairo,  Egypt,  the  leafless  period  is  only  two  months 
long,  and  in  very  warm  and  moist  climates  the  vines  are  ever- 
green. So,  too,  cherry  trees  are  evergreen  in  Ceylon,  and 
beeches  in  Madeira. 

A  large  shrubby  Euphorbia*  common  in  southern  Italy,  is 
found  absolutely  leafless  during  July  and  August,  when  grow- 
ing on  the  faces  of  limestone  cliffs.  But  in  moist  soil,  within 

a  stone's  throw  of  the 
leafless  plants,  there 
may  be  found  others 
profusely  leafy. 

443.  Halophytes.  A 
halophyte  is  a  plant 
which  can  thrive  in  a 
soil  containing  much 
common  salt  or  other 
saline  substances.  The 
seaside  is  the  principal 
region  of  halophytic 
vegetation,  but  many 
halophytic  plants  are 


FIG.  363.   The  mangrove,  a  halophyte 
After  W.  M.  Davis 

also  to  be  found  in  the  neighborhoods  about  salt  springs  and  the 
"  alkali "  lands  of  the  Southwest  and  the  Pacific  slope. 

The  mangrove  tree  (Fig.  363)  is  one  of  the  most  remarkable 
of  halophytes.  It  grows  in  shallow  water  along  the  seashore, 
and  sends  out  many  aerial  roots  which  at  length  find  their  way 
down  into  the  salt  mud.  In  this  way  it  collects  drift  material 
and  gradually  extends  the  shore  line  farther  out  to  sea. 

1  E.  dendroides. 


HALOPHYTES  469 

444.  Form  and  structure  of  halophytes.    Most  halophytes 
present  certain  peculiarities  of  form  and  structure,  such  as  suc- 
culence of  stem  or  leaves,  or  both,  a  highly  developed  palisade 
layer,  small  intercellular  spaces,  diminution  of  evaporating  sur- 
face, and  often  specially  developed  tissue  for  storage  of  water. 
These  are  evidently  xerophytic  characteristics,  and  their  pres- 
ence may  be  due  to  two  causes : 

First,  the  occurrence  of  salt  in  the  soil  renders  absorption  of 
the  soil  water  comparatively  difficult,  since  osmosis  takes  place 
more  readily  between  nearly  pure  water  and  the  liquid  con- 
tents of  the  young  roots  and  root  hairs  than  between  salt  water 
and  the  liquids  within  the  root.  Halophytes  may  therefore  be 
on  a  short  allowance  of  water  even  when  their  roots  are  con- 
stantly wet. 

Second]  the  absorption  of  much  salt  would  poison  the  plant, 
and  therefore  it  is  an  advantage  to  keep  down  transpiration 
and  with  it  the  rate  at  which  salt  water  is  allowed  to  enter 
the  roots. 

445.  Halophytes  not  dependent  on  salts.    It  is  worth  while 
to  note  the  fact  that  halophytes  are  not  usually  dependent  on 
a  highly  saline  soil.    They  are  salt  tolerators  rather  than  salt 
lovers.1    But  they  nourish  in  saline  localities  because  they  are 
capable  of  enduring  much  more  salt  than  ordinary  plants,  and 
so  can  grow  in  salt  marshes  and  such  localities  comparatively 
unhindered  by  the  competition  of  non-halophytic  species. 

446.  Other  kinds  of  ecological  classes.    One  may  class  plants 
with  reference  to  their  habits  in   many   other   regards    than 
according  to  their  relative  economy  of  water  or  their  tolerance 
of  salts.  Only  one  other  kind  of  classification  need,  however,  be 
mentioned  in  this  chapter, —  that  is,  the  division  into  sun-loving 
and  shade-loving  plants.    Even  in  very  dense  forests  some  plants 
are  found  growing  on  the  soil  in  the  twilight  formed  by  the 
shade  of  the  trees.    Some  of  this  undergrowth  is  of  seed  plants, 

1  Or,  in  technical  terms,  the  plants  which  grow  in  saline  soils  are  facul- 
tative halophytes  but  i\ot%obligate  halophytes. 


470 


ECOLOGICAL  GROUPS 


and  there  are  many  ferns  and  mosses  which  nourish  in  such 
situations.  Shade  plants  commonly  have  large  pale  leaves,  and 
generally  (except  in  ferns)  the  leaves  are  not  much  cut  or  lobed 
(Fig.  364,  A).  Sun-loving  plants,  on  the  other  hand,  usually 

have  comparatively 
little  leaf  surface,  and 
the  leaves  are  often  cut 
into  narrow  divisions 
(Fig.  364,  B).  Appar- 
ently the  broad  leaf 
surfaces  in  the  one 
class  are  to  expose 
many  green  cells  to  the 
light  for  starch  making, 
while  in  the  other  class 
the  slender  leaf  divi- 
sions expose  enough  as- 
similating cells,  and  at 
the  same  time  the  nar- 
rowness of  the  division 
permits  plenty  of  light 
to  penetrate  to  the 
plant's  lower  leaves.  It 
is  also,  doubtless,  much 
easier  for  leaves  like 
those  of  the  yarrow, 
the  dog  fennel,  the 
tansy,  the  carrot,  and 
their  like  to  withstand  the  action  of  severe  winds,  to  which  they 
are  often  exposed,  than  it  would  be  for  leaves  like  those  of 
the  jack-in-the-pulpit,  the  trilliums,  the  lily  of  the  valley,  and 
similar  leaves. 

447.  Sun  leaves  and  shade  leaves  on  the  same  plant.  On 
plants  of  the  same  species,  or  even  on  the  same  individual,  sun 
leaves  and  shade  leaves  often  differ  widely-.  On  comparing  the 


FIG.  364 

A,  a  shade  plant  (Clintonia) ;  B,  a  sun  plant, 
dog  fennel  (Maruta) 


SUN  LEAVES  AND  SHADE  LEAVES       471 

leaves  from  the  exterior  and  the  interior  of  the  crown  of  a  de- 
ciduous tree,  or  such  an  evergreen  species  as  the  live  oak  or  the 
olive,  the  sun  leaves  are  usually  found  to  be  lighter-colored,  of 
smaller  area,  thicker,  and  of  more  xerophytic  structure  than  the 
shade  leaves.  The  difference  in  size  may  be  very  great,  the 
smallest  sun  leaves  sometimes  not  covering  more  than  a  tenth 
of  the  area  of  the  largest  shade  leaves  on  the  same  plant.  There 
is  usually,  also,  a  notable  difference  in  the  form  of  the  two 
kinds,  the  sun  leaves  being  narrower  in  proportion  to  their 
length.  Sun  leaves  are  often  several  times  as  thick  as  shade 
leaves,  and  have  far  more  completely  developed  palisade  layers. 
The  latter  may  even  (in  leaves  grown  in  dense  shade)  be  quite 
lacking,  and  the  regular  palisade  cells  be  replaced  by  loosely 
arranged,  funnel-shaped  cells,  with  their  broader  ends  toward 
the  epidermis.  Sun  leaves  have  a  much  stronger  fibre-vascular 
framework  than  those  developed  in  a  comparatively  feeble  light. 
In  the  ease  of  plants  which  have  the  leaves  more  or  less  hairy 
or  scaly,  the  covering  of  these  epidermal  outgrowths  is,  as  might 
be  expected,  much  more  dense  on  the  sun  leaves. 

Probably  the  work  of  all  kinds  done  by  the  sun  leaves  is  far 
greater  than  that  done  by  shade  leaves  of  the  same  species. 
This  is  partly  due  to  the  much  greater  supply  of  energy  daily 
received  by  the  former  from  the  sun,  and  it  is  also  due  to  their 
more  capacious  conducting  system  and  greater  supply  of  chloro- 
plasts.  The  transpiration  of  a  given  area  of  sun  leaves  is  at 
times  tenfold  that  of  the  same  area  of  shade  leaves  (both  being 
placed  for  the  time  in  full  sunlight). 

448.  Transition  of  a  plant  from  shade  conditions  to  sun  con- 
ditions. It  is  characteristic  of  many  kinds  of  forest  trees  that 
the  young  seedlings  are  much  more  tolerant  of  dense  shade  than 
the  adult  trees.  Sometimes  their  seeds  will  hardly  germinate 
at  all  unless  thoroughly  shaded,  and  the  young  trees  for  the 
first  few  years  flourish  best  in  the  shade.  Afterwards  most  trees 
need  a  good  deal  of  sunlight,  but  they  may  live  long  with  a 
scanty  supply  of  light.  ''Ihe  red  spruce  sometimes  lingers  on  for 


FIG.  365.   An  epiphytic  fern  (Platycerium)  on  a  tree  trunk 

The  more  upright  leaves  next  to  the  trunk  of  the  tree  serve  to  collect  moisture  and 
to  accumulate  a  deposit  of  decaying  vegetable  matter,  while  the  outer  leaves 
serve  as  foliage  and  bear  spores.  —  After  Schimper 


472 


WATER  SUPPLY  OF  EPIPHYTES  473 

fifty  or  a  hundred  years,  reaching  meantime  a  diameter  of  not 
more  than  two  inches,  and  then,  on  getting  more  light,  shoots 
up  into  a  large  and  valuable  timber  tree.1 

449.  Epiphytes.    It  is  even  easier  for  a  plant  to  secure  enough 
sunlight  in  a  forest  region  by  perching  itself  upon  the  trunk, 
branches,  or  leaves  of  a  tree  than  by  climbing,  as  our  wild  grape- 
vines and  the  great  tropical  lianas  do.    There  is  a  large  number 
of  such  perched  plants,  or  epiphytes  (meaning  upon  a  plant),  par- 
tictdarly  in  such  tropical  forests  as  those  of  Fig.  39  and  Plate 
XIII.    Epiphytic  forms  occur  among  many  different  groups  of 
seed  plants  and  of  spore  plants,  especially  lichens.    The  stag- 
horn  fern,  shown  in  Fig.  365,  is  a  good  example  of  an  epiphyte. 
Instances  among  seed  plants  are  the  so-called  Florida  or  Spanish 
moss  (Plate  III)  and  orchids  like  those  in  Fig.  13. 

450.  Water  supply  of   epiphytes.    Epiphytes    secure   their 
supply  of  water  and  dissolved  salts  in  several  different  ways, 
some  through  roots  by  absorption  from  the  moist  bark  on  which 
they  grow,  others  by  sending  roots  down  until  they  reach  the 
earth,  others  by  means  of  a  network  of  aerial  roots  fully  exposed 
to  the  air,  —  as  in  the  orchid  just  mentioned,  —  and  still  others 
by  means  of  leaves  which  function  as  roots.    Some  species,  like 
the  Florida  moss,  absorb  water  very  rapidly  from  dew  or  rains, 
while  others,  as  the  stag-horn  fern  .(Figs.  272,  365),  and  Til- 
landsia  bulbosa,  a  relative  of  the  Florida  moss,  hold  water  in 
reservoirs  at  the  bases  of  the.  leaves,  with  or  without  the  aid  of 
spongy  decaying  vegetable  matter.    From  the  great  vicissitudes 
in  their  water  supply  most  epiphytes  among  seed  plants  possess 
xerophytic  characteristics. 

1  See  the  Primer  of  Forestry,  Part  I,  United  States  Department  of  Agri- 
culture, 1899,  pp.  33-35. 


CHAPTER   XXXVII 
PLANT  FORMATIONS;  ZONATION* 

451.  Plant  formations.    One  of  the  first  things  which  the 
young  field  botanist  learns  is  the  fact  that  the  distribution  of 
plants  depends  largely  on  the  character  of  the  ground  they  occupy. 
There  is  in  any  small  territory,  such  as  a  Count}',  for  example, 
one  assemblage  of  plants  for  the  waters  of  ponds  and  another 
for  their  shores,  one  for  swamps,  one  for  moderately  dry  uplands, 
one  for  very  dry  hilltops,  and  so  on.    The  aquatic  plants  of  the 
sea  are  very  different  from  those  of  fresh  water.    Sandstone  and 
limestone  soils  have  vegetations  peculiar  to  themselves;1  the 
long-leaved  pine,  the  scrub  pine,  and  the  chestnut  are  character- 
istic trees  of  sandy  soils,  while  most  of  the  oaks,  the  hackberries, 
and  the  black  walnut  are  generally  found  in  limestone  regions. 

The  collection  of  plants  as  found  in  any  given  kind  of  station 
or  habitat,  especially  when  prominent  and  well  defined,  is  called 
&  formation.  Thus  we  have  marine  aquatic  formations,  sea-beach 
formations,  pond  formations,  bog  formations,  sand-hill  forma- 
tions, meadow  formations,  heath  formations,  forest  formations, 
and  many  others  such  as  the  student  may  designate  for  himself. 

452.  Plant  associations.    Usually  the  plant  formation  is  divis- 
ible into  assemblages  or  unit  groups,  which  are  much  more  alike 
in  their  vegetation  than  is  the  formation  as  a  whole.    Thus  a 
woodland  formation  may  consist  of  pine  patches,  oak  patches,  and 

*  To  THE  INSTRUCTOR  :  If  it  is  necessary  to  cut  down  the  discussion  of 
these  topics  to  little  more  than  definitions,  only  the  first  three  sections  of  the 
chapter  need  be  read. 

1  Perhaps  this  is  sometimes  due  to  physical  rather  than  to  chemical  causes. 
In  other  words,  the  chemical  differences  in  soils  are  usually  accompanied  by 
differences  in  their  porosity,  their  capacity  for  retaining  water,  for  absorbing 
heat  from  the  sun's  rays,  and  so  on,  which  greatly  modify  their  effect  on  plants. 

474 


ZOtfATION  475 

birch  patches.  A  grass-land  formation  may  consist  of  patches  of 
timothy  and  others  of  redtop,  and  so  on.  Such  minor  groups 
are  often  called  associations. 

In  some  cases  it  may  be  possible  to  show  that  the  association 
is  based  on  the  mutual  relations  to  each  other  of  the  plants 
which  compose  it,  while  the  formation  as  a  whole  depends  on 
the  characteristics  of  the  station  in  which  it  exists,  i.e.  on  soil, 
climate,  and  so  on..1 

453.  Zonation.  The  most  striking  occurrence  of  plant  forma- 
tions is  in  localities  where  sharply  contrasted  conditions  of  life 
exist  side  by  side.  It  is  often  possible,  within  the  radius  of  a 
few  hundred  feet,  to  travel  from  the  floating  aquatic  vegetation 
of  the  deeper  waters  of  a  pond,  through  the  rooted  aquatic  forms 
of  the  shallower  water,  to  the  sub-aquatic  species  of  the  wet 
shore,  then  past  the  sand-loving  plants  of  the  sand  dunes  farther 
back  from  the  water,  and  finally  into  the  wood  or  meadow  vege- 
tation of  ordinary  soil.  Such  a  series  of  zones  is  shown  in 
Fig.  366  and  in  Plate  X. 

Similar  diagrams  may  be  made  to  illustrate  the  distribution 
of  plants  about  a  salt  spring  or  pool,  along  the  seashore  or  the 
margin  of  a  salt  marsh,  on  the  top  and  sides  of  an  isolated  hill 
with  a  dry,  ledgy,  or  sandy  summit,  or  even  about  an  old  unused 
gravel  pit  or  a  railroad  embankment.  Less  clearly  defined  but 
very  interesting  and  extensive  zones  may  be  studied  with  rela- 
tion to  submerged  aquatics,  particularly  among  marine  plants,  as 
shown  in  Fig.  202. 

Among  the  most  striking  and  symmetrical  instances  of  zona- 
tion  are  those  to  be  found  about  the  salt  marshes  of  some  of 
the  deserts  of  the  far  West.  The  waters  of  these  marshes  are 
too  salt  to  support  vegetation,  but  encircling  their  borders  may 
sometimes  be  found  as  many  as  six  broad  concentric  bands  of 
abundant  vegetation. 

1  Some  authors  use  the  term  association  as  an  equivalent  for  the  term 
formation  as  here  employed.  Consocies  is  sometimes  used  with  the  same 
meaning  as  is  here  given  to"  association. 


•5cm. 


-500ft, 


FIG.  366.   Zonation  about  a  pond 

I,  pond ;  II,  bog  zone ;  III,  swampy  thicket  zone ;  IV,  incomplete  zone  in  arid  soil 
of  a  sand  pit;  V,  dry  meadow  zone;  VI,  dry  woodland  zone ;  a,  floating  mass 
of  Eriocaulon ;  b,  deepest  area  in  pond,  six  to  ten  feet;  c,  association  of  tall 
rushes  (Juncus  militaris) ;  d,  birch  woodland  (Betula  populifolid) ;  WE,  line 
intersecting  formations  of  west  side  of  pond ;  SN,  line  intersecting  forma- 
tions of  north  end  of  pond 

476 


ZONATION  ABOUT  A  POND  477 

454.  Zonation  in  Fig.  366.    The  diagram  represents  some  of 
the  principal  formations  observed  in  and  about  a  rather  shallow 
pond  in  eastern  Massachusetts  in  October,  1905. 

The  pond  was  almost  encircled  by  six  zones  of  vegetation, 
only  one  of  which  (the  bog  zone)  entirely  surrounded  the  water. 
All  the  zones  except  that  of  the  meadow  consisted  of  wild  spe- 
cies, growing  under  nearly  natural  conditions. 

The  pond  itself  had  a  rather  scanty  algal  vegetation,  among 
which  the  most  noticeable  forms  were  a  green  alga  (Bulbochcete), 
related  to  (Edogonium,  and  a  blue-green  alga  (Ccelosplicerium). 

Seed  plants  were  well  represented  in  the  waters  of  the  pond. 
In  the  deepest  portion,  at  b,  no  seed  plants  were  found  extend- 
ing to  the  surface,  but  there  were  many  young  specimens  of  a 
rush  (Juncus.  militaris)  with  filiform  leaves  deeply  submerged.  In 
general  the  pond  was  populated  by  pond  lilies  (NympJicea),  cow 
lilies  (Nuphar),  three  or  more  species  of  potidweed  (Potamoge- 
tori),  with  much  pipewort  (Eriocaulori)  and  duckweed  (Lemna). 
In  the  shallowest  portions,  usually  in  six  inches  or  less  of  water, 
were  found  some  six  other  herbaceous  species. 

The  spermatophytic  vegetation  of  the  pond  may  be  divided 
according  to  its  mode  of  growth  into  classes  as  follows : 

Floating  plants •     .  Lemna 

Plants  which  grow  rooted  and  submerged      .      .        Potamogeton 

(  Juncus 

Plants  which  grow  rooted,  but  with  more  or  less    j  Pontederia 
of  the  stem  or  leaf  surface  in  air I  Nymphcea 

[  Nuphar 

Plants  of  shallow  water  (six  inches  or  less),  or    C 

,  .  ,  n     A .  f.     TT       ,,  I  hleochanx 

which  grow  on  floating  rafts  like  that  at  a,    4  „     . 

•j.1  f  ,!  •  T  Xyris.  etc. 

with  most  of  the  plant  body  aerial    .....{_ 

455.  Contents  of  the  zones.    It   would   involve   too   much 
detail    to    enumerate   the    species    of   the   several  land   zones 
(II-VI),  but  they  may  be  briefly  summarized  as  follows : 

The  bog  zone  contained  some  twenty-one  conspicuous  species, 
especially  peat  moss  and  herbaceous  seed  plants. 


478  PLANT   FORMATIONS 

The  swampy  thicket  zone  contained  mostly  shrubs  and  small 
trees,  including  an  alder,  a  blueberry,  a  pepper  bush  (Clethra), 
gray  birch,  and  red  maple. 

The  arid  soil  zone  contained  more  than  twenty  species,  mostly 
sand-frequenting  annual  seed  plants. 

The  meadow  was  growing  under  artificial  conditions,  and  it 
was  merely  noted  that  its  flora  consisted  mainly  of  cultivated 
grasses. 

The  dry  woodland  zone  contained  some  twenty-three  con- 
spicuous forms.  The  three  principal  trees,  in  the  order  of  their 
numbers  (omitting  the  region  d),  were  white  pine,  northern  pitch 
pine,  and  red  oak.  The  forest  floor  contained  an  abundant  growth 
of  shrubs  and  herbs.  At  least  five  species  of  the  latter  were 
common  to  the  woodland  zone  and  the  arid  sand  zone. 

The  marked  differences  in  the  character  of  the  vegetation 
of  the  several  zones  were  almost  wholly  due  to  differences  in 
the  amount  of  water  supply.  Not  only  would  the  trees  have 
died  if  transplanted  into  the  pond,  or  the  pond  aquatics  have 
died  if  removed  to  the  dry  sandy  soil  of  the  woodland,  but  in 
general  each  set  of  plants  was  better  off  in  its  own  zone  than  it 
would  have  been  in  any  other.  The  sand-pit  flora  was,  however, 
only  a  short-lived  succession,  soon  to  be  followed  by  the  wood- 
land flora. 

456.  Similar  vegetation  due  to  similar  conditions.  As  soon 
as  one  begins  to  collect  plants  in  a  set  of  localities  new  to  him, 
he  often  discovers  that  his  old  acquaintances  are  still  to  be  found 
grouped  as  he  has  been  accustomed  to  see  them.  The  muddy 
borders  of  ponds  from  Maine  to  Minnesota  and  beyond  are 
fringed  with  the  same  kinds  of  bur  reeds  and  sedges,  set  with 
water  plantain  and  decorated  with  the  soft  white  blossoms  of 
the  arrowhead.  The  sand  dunes  along  the  northern  Atlantic 
coast  and  those  that  border  Lake  Michigan  are  clothed  with  a 
sparse  vegetation,  which  in  both  cases  includes  the  little  beach 
plum,  such  coarse  grasses  as  that  shown  in  Plate  I,  and  the 
straggling  sea  rocket.  Barnyards  and  waste  grounds  about  farm 


SIMILAR  SPECIES   REPLACE  EACH  OTHER          479 

buildings  from  Massachusetts  to  Missouri  contain  such  weeds 
as  the  dog  fennel,  the  low  mallow  ("  cheeses "),  motherwort, 
catnip,  and  some  smartweeds. 

A  little  study  of  such  cases  soon  leads  one  to  the  conclusion 
that  these  plant  associations  and  multitudes  of  others  exist 
because  all  the  plants  in  each  association  are  adapted  to  their 
special  environment.  Wherever  such  an  environment  occurs 
such  an  association  will  be  found  in  it,1  or,  if  not  already  there, 
will  nourish  when  introduced. 

457.  Similar  species  replace  each  other.  Two  sets  of  locali- 
ties alike  in  many  respects  but  unlike  in  some  points  are  often 
inhabited  by  different  species  of  the  same  genus.  For  instance, 
the  pine  barrens  of  New  England  and  the  adjacent  states  are 
commonly  covered  with  the  northern  pitch  pine,2  while  far 
southward,  in  sandy  soil,  its  place  is  taken  by  the  long-leaved 
pine.3  Along  streams  and  swamps  northward  the  speckled  alder  4 
is  generally  found,  while  southward  the  smooth  alder 5  is  most 
common.  In  rich  w^oods  of  the  northeastern  United  States  the 
painted  trillium  6  and  the  erect  trillium  ("  Benjamin,"  or  "  squaw 
root ") 7  are  the  commonest  species,  while  farther  south,  in  sim- 
ilar localities,  the  sessile  trillium,8  Underwood's  trillium,9  and 
the  large-flowered  trillium10  are  abundant. 

In  all  such  cases  —  and  they  are  very  numerous  —  we  are 
to  infer  that  the  genus  is  peculiarly  well  adapted  to  some  spe- 
cial set  of  conditions,  as  sandy  soil,  brooksides,  or  the  rich, 
shaded  soil  of  woodlands.  The  more  northerly  species  are  capa- 
ble of  enduring  the  severe  winters  and  brief  summers  of  their 
region,  while  the  more  southerly  ones  perhaps  cannot  do  so. 
The  relative  warmth  of  the  climates  in  which  they  live  may 
not  be  the  only  reason,  or  even  the  principal  reason,  for  the 


1  That  is,  in  localities  not  separated  by  such  natural  barriers  as  seas, 
high  mountains,  or  deserts. 

2  Pinus  rigida.  5  A.  serrulata.  8  T.  sessile. 

3  P.  palustris.  6  Trillium  erythrocarpum.  9  T.  Underwoodii. 

4  Alnus  incana.  7  T.  erectum.  10  T.  grandiflorum. 


480  PLANT   FORMATIONS 

distribution  of  such  plants  as  those  just  mentioned,  but  it  is 
one  factor,  at  any  rate,  and  it  is  certain  that,  on  the  whole,  most 
of  our  native  and  thoroughly  naturalized  plants  are  growing 
under  what  is,  for  them,  the  best  environment,  since  they  can- 
not usually  be  made,  to  exchange  places  with  one  another.  If  a 
square  mile  of  land  in  Louisiana  were  to  be  planted  with  Min- 
nesota species,  and  a  square  mile  in  Minnesota  with  Louisiana 
species,  it  is  very  improbable  that  either  tract,  if  left  to  itself, 
would  long  retain  its  artificial  flora.  To  this  rule  there  are, 
however,  important  exceptions. 

458.  Formations  of  few  species.  It  is  not  uncommon  to  find 
tracts  of  land  or  water  inhabited  by  great  numbers  of  seed 
plants  of  the  same  species,  so  as  almost  to  exclude  all  other  vege- 
tation except  microscopic  spore  plants.  Ponds  and  slowly  flow- 
ing streams  are  often  filled  in  this  wray  with  the  water  hyacinth,1 
the  water  cress,  or  the  American  lotus.2  The  canebrakes  of  the 
South  and  the  wild  rice  swamps  along  northern  lakes  arid  rivers 
are  other  examples  of  an  extremely  simple  flora  spread  over 
large  areas.  Prairies  not  infrequently  for  many  square  miles 
are  covered  mainly  (not  entirely)  with  a  very  few  kinds  of 
grasses.  The  arid  plains  of  the  Eocky  Mountain  region,  over 
thousands  of  square  miles,  contain  little  vegetation  except  sage- 
brush (Artemisia  tridentata),  and  immense  tracts  of  snow  in 
the  arctic  regions  are  destitute  of  plant  life  except  for  the 
red-snow  alga  (Splicerella  nivalis,  Sec.  215),  by  which  they  are 
colored  pink. 

In  all  such  cases  it  is  evident  that  the  single  species  or  the 
few  species  which  populate  the  area  can  endure  the  conditions 
of  existence  there  so  well  that  other  plants  which  migrate  into 
their  territory  cannot  compete  with  them. 

1  Eichhornia.  2  Nelwnbo. 


CHAPTER    XXXVIII 

PLANT  GEOGRAPHY  * 

459.  Regions  of  vegetation.    The  earth's  surface  (that  of  the 
land)  has  been  described  by  one  of  the  greatest  of  geographical 
botanists 1  as  divided  into  twenty -four  regions   of  vegetation. 
His  grouping  takes  account  of  all  the  principal  continental  areas 
which  have  a  characteristic  set  of  plants  of  their  own,  as  well 
as  of  the  most  important  islands.    But  a  simpler  arrangement 
is  to  consider  the  plant  life  of  the  earth  as  distributed  among 
the  following  regions : 

1.  The  tropical  region.  3.  The  arctic  regions. 

2.  The  temperate  regions.  4.  Mountain  heights. 

5.   Bodies  of  water. 

Any  good  geography  gives  some  account  of  at  least  the 
land  vegetation  of  the  earth.  It  is  only  necessary  in  the  pres- 
ent chapter  to  point  out  a  few  of  the  most  important  charac- 
teristics of  the  plants  of  the  areas  mentioned  above  and  to  give 
some  reasons  why  the  plant  population  of  each  has  its  special 
characteristics. 

460.  Tropical   vegetation.    Within   the  tropics  two  of   the 
great  factors  of  plant  life  and  growth,  namely,  light  and  heat, 
are  found  in  a  higher  degree  than   elsewhere-  on  the  earth. 
Moisture,  the  third  requisite,  is  in  some  regions  very  abundant 
(over  forty  feet  of  rainfall  in  a  year),  or  sometimes,  in  desert 
areas,  almost  lacking.    We  find  here,  accordingly,  the  greatest 
extremes  in  amount  of  vegetation,  from  the  bare  sands  or  rocks 
of  the  Sahara  desert  (Fig.  367)  to  the  densely  wooded  basin  of 

*  To  THE  INSTRUCTOR  :  Unless  the  present  chapter  can  he  discussed  in 
considerable  detail,  it  might  better  be  omitted  than  hastily  dealt  with. 
1  A.  Grisebach,  in  Die  Vegetation  der  Erde. 

481 


482  PLANT   GEOGRAPHY 

the  Kongo  and  of  the  Amazon.  The  rainy  forests  of  the  tropics 
contain  extraordinary  numbers  of  species.  For  example,  near 
Lagoa  Santa  in  Brazil,  in  an  area  of  three  square  miles,  there  are 
found  about  four  hundred  species  of  trees.  Xerophytic  plants, 
many  of  them  with  extremely  complete  adaptations  for  support- 
ing life  for  long  periods  without  water,  are  characteristic  of 
tropical  deserts,  while  many  of  the  most  decided  hydrophytes 
among  land  plants  are  found  in  the  dripping  sub-tropical  forest 


in. 

FIG.  367.    Hills  of  drifted  sand  in  the  Sahara 
After  W.  M.  Davis 

interiors.  Throughout  a  large  part  of  the  belt,  reaching  five 
degrees  each  way  from  the  equator,  there  are  daily  rains  the 
year  round. 

461.  Vegetation  of  temperate  regions.  We  are  all  familiar  in 
a  general  way  with  the  nature  of  the  plant  life  of  the  north  tem- 
perate zone ;  that  of  the  south  temperate  is  in  most  ways  similar 
to  our  own.  Most  of  the  annuals  and  biennials  are  of  a  medium 
type,  not  decided  xerophytes  nor  hydrophytes,  and  the  peren- 
nials are  mainly  tropophytes.  There  are  no  desert  areas  so  large 
or  so  nearly  destitute  of  plants  as  those  found  in  sub-tropical 
regions,  neither  are  there  any  such  luxuriant  growths  as  occur  in 
the  rainy  forest  regions  of  the  tropics.  On  the  other  hand,  the 
largest  trees  on  earth,  the  giant  redwoods,  or  Sequoias  (Fig.  33), 
occur  in  the  temperate  portion  of  North  America,  along  the 
Sierra  Nevada,  and  the  taller,  though  less  bulky,  gum  trees 
(Eucalyptus)  of  Australia  grow  in  a  warm  temperate  region. 


ASSOCIATIONS  DUE   TO   CONDITIONS  OF  SOIL      483 


462.  Temperate  plant  associations  due  to  special  conditions 
of  soil.  Even  where  the  climate  is  a  moderate  one  as  regards 
temperature  and  rainfall,  peculiar  soils  may  cause  the  assemblage 
of  exceptional  plant  associations.  Some  of  the  most  notable  of 
such  associations  in  temperate  North  America  are  those  of  the 
salt  marshes,  the  sand  dunes,  and  the  peat  bogs. 

In  salt,  marshes  the  water  supply  is  abundant,  but  plants  do 
not  readily  absorb  salt  water  by  their  roots,  so  that  the  plants 
which  grow  in  salt  marshes  usually 
have  something  of  the  structure  and 
appearance  of  xerophytes.  Some  of 
them  are  fleshy  (Fig.  368),  and  some 
species  are  practically  leafless. 

Sand  dunes,  whether  along  the 
seacoast  or  near  the  Great  Lakes, 
offer  a  scanty  water  supply  to  the 
roots  during  much  of  the  year,  and 
the  soil  water  contains  less  of  the 
raw  materials  for  plant  food  than  is 
offered  by  that  of  ordinary  soils. 
Many  grasses  thrive,  however,  in 
these  shifting  sands  (Plate  I),  and 
some,  like  the  beach  grass  (Am- 
mophila)  of  the  Atlantic  coast  and 
the  Great  Lakes,  will  continue  to  grow  upward  as  the  sand  is 
piled  about  them  by  the  winds,  until  they  have  risen  to  a  level 
of  a  hundred  feet  above  the  starting  point. 

The  water  of  peat  bogs  contains  little  mineral  matter,  and 
only  a  very  scanty  supply  of  nitrogen,  in  the  form  of  nitrates 
dissolved  in  it.  The  bog  plants,  therefore,  must  either  get  on 
with  an  exceptionally  small  supply  of  nitrogen,  or  they  must 
get  it  from  an  unusual  source.  The  peat  mosses  adopt  the 
former  alternative,  while  the  sundews,  the  pitcher  plants,  and 
some  other  species  adopt  the  latter  and  derive  their  nitrogen 
supply  largely  from  insects  which  they  catch,  kill,  and  digest. 


FIG.  3(38.   A  halophyte 
(Salicornia) 


484  PLANT   GEOGRAPHY 

463.  Arctic  vegetation.  The  seed  plants  of  the  arctic  flora 
are  mostly  perennials,  never  trees,  though  many  of  the  species, 
as  the  willow,  alder,  and  birch,  belong  to  groups  that  are  trees 
in  other  regions.  By  the  large  bulk  of  the  underground  por- 
tion as  compared  with  that  of  the  part  above  ground,  they  are 
adapted  to  a  climate  in  which  they  must  lie  dormant  for  not 
less  than  nine  months  of  the  year.  The  flowers  are  often  showy 
and  appear  very  quickly  after  the  brief  summer  begins.  Mosses 


FIG.  369.   A  plant  of  arctic  willow 
About  natural  size 

and  lichens  are  abundant,  —  the  latter  of  economical  importance 
because  they  furnish  a  considerable  part  of  the  food  of  reindeer. 
464.  Mountain  or  alpine  vegetation.  In  a  general  way  the 
effect  of  ascending  a  mountain,  so  far  as  vegetation  is  concerned, 
is  like  that  of  traveling  into  colder  regions.  It  was  long  ago  sug- 
gested in  regard  to  Mount  Ararat,  that  on  ascending  it  one  trav- 
ersed first  an  Armenian,  then  a  south  European,  then  a  French, 
then  a  Scandinavian,  and  finally  an  arctic  flora.  Up  to  a  certain 
height,  which  varies  in  different  latitudes,  the  slopes  of  moun- 
tains are  very  commonly  forest-covered.  The  altitude  up  to 


MOUNTAIN   OR  ALPINE  VEGETATION  485 

which  trees  can  grow,  or  as  it  is  commonly  called  in  this  coun- 
try the  "  timber  line,"  is  somewhat  over  twelve  thousand  feet 
in  the  equatorial  Andes,  and  lessens  in  higher  latitudes  as  one 
goes  either  way  from  the  equator,  until  in  the  arctic  regions  it 
reaches  sea  level.  In  the  White  Mountains,  for  instance,  the 
timber  line  only  rises  to  about  forty-five  hundred  feet.  The  seed 
plants  of  alpine  regions  in  all  parts  of  the  earth  have  a  peculiar 
and  characteristic  appearance.  It  is  easiest  to  show  how  such 


FIG.  370.   Trees  near  the  timber  line  on  the  slope  of  Pikes  Peak 
After  W.  M.  Davis 

plants  differ  from  those  of  the  same  species  as  they  look  when 
growing  in  ordinary  situations  by  reference  to  the  plants  them- 
selves or  to  good  pictures  of  them  (see  Fig.  372).  The  differences 
between  alpine  and  non-alpine  plants  of  the  same  or  closely 
related  species  have  been  summed  up  as  follows 1 :  "  The  alpine 
individuals  have  shorter  stems,  smaller  leaves,  more  strongly 
developed  roots,  equally  large  or  somewhat  larger  and  usually 
somewhat  more  deeply  colored  flowers,  and  their  whole  structure 
is  drought-loving  (xerophilous)."j 

1  By. A.  F.  W.  Schimper. 


486 


PLANT   GEOGRAPHY 


Trees  at  great  elevations  are  stunted  and  gnarled  by  scanty 
nutrition  and  pressure  of  wind  and  snow  (Fig.  370). 


FIG.  371.    Decrease  in  size  of  trees  at  high  elevations  (Canadian  Rockies) 

Where  the  prevailing  winds  come  mainly  from  one  quarter, 
all  the  trees  of  considerable  areas  may  be  inclined  strongly  in 
one  direction,  as  in  Plate  XL1 

1  This  phenomenon  is  also  very  noticeable  along  many  coasts. 


a  « 


MOUNTAIN  OR  ALPINE  VEGETATION 


487 


The  gradual  diminution  of  the  height  of  the  trees  on  ascend- 
ing a  mountain  is  well  shown  in  Fig.  3  7 1.1  The  treeless  charac- 
ter of  the  mountain  summit  is  also  plain. 

Recent  experiments  have  shown  that  many  ordinary  plants 
promptly  take  on  alpine  characteristics  when  they  are  transferred 
to  moderate  heights  on  mountains.  For  instance,  a  rather  com- 
monly cultivated  sunflower,2  when  planted  at  a  height  of  about 
sixty-five  hundred 
feet,  instead  of 
having  a  tall,  leafy 
stem,  produces  a 
rosette  of  very  hairy 
leaves  lying  close  to 
the  ground,  thus  be- 
coming almost  un- 
recognizable as  a 
sunflower.  The 
change  is  even 
greater  than  that 
shown  in  the  rock 
rose  (Fig.  372)  culti- 
vated by  the  same 
experimenter.  The 
peculiarities  of  alpine 
plants  appear  to  be 
due  mainly  to  the  in- 
tense  light  which 
they  receive  during  the  daytime,3  to  the  strongly  drying  char- 
acter of  the  air  in  which  they  grow  (due  partly  to  its  rarefaction), 
and  to  the  low  temperature  which  they  must  endure  at  night. 

1  Part  of  the  diminution  is  only  apparent,  —  the  effect  of  distance,  —  but 
the  growth  at  the  highest  levels  is  often  less  than  waist  high. 

2  Helianthus  tuberosus,  the  so-called  Jerusalem  artichoke. 

3  The  experiments  of  Professor  Frederic  E.  Clements  on   Pikes  Peak, 
however,  seem  to  show  that  light  is  not  a  principal  factor  in  the  production 
of  alpine  characteristics  in  plants. 


FIG.  372.  Two  plants  of  rock  rose  (Helianthemum) 

A,  low  ground  form;  B,  alpine  form.    Both  drawn 
to  the  same  scale 


488  PLANT  GEOGRAPHY 

465.  Aquatic  vegetation.    Plants  which  live  wholly  in  water 
often  need  a  less  complicated  system  of  organs  than  land  plants. 
True  roots    may  be    dispensed  with   altogether,  as  in  many 
seaweeds,  in  most  fresh-water  algae,  and  in  some  seed  plants. 
Many  such  plants  have  mere  holdfasts  that  keep  them  from 
being  washed  out  of  place.     In  the  duckweeds  (Fig.  355)  the 
roots  answer  the  purpose  of  a  keel  and  keep  the  flat  expanded 
part  of  the  plant  from  turning  bottom  up.    The  tissues  that 
serve  to  strengthen  the  plant  body  are  not  much  developed  in 
submerged  aquatics,  since  the  water  supports  most,  if  not  all, 
of  the  weight  of  the  plant.    Stomata  are  absent,  and  the  absorp- 
tion of  carbon  dioxide  and  giving  off  of  oxygen  go  on  directly 
through  the  delicate  cell  walls,  unprotected  by  an  epidermis 
(Fig.  354,^).    Submerged  aquatic  seed  plants  occur  in  consider- 
able abundance  in  sea  water  as  well  as  in  fresh  waters,  but  the 
marine  forms  do  not  include  many  species. 

466.  Influence  of  rainfall  in  determining  regions  of  vegeta- 
tion.   While  the  mean  annual  temperature  and  the  extremes  of 
heat  and  cold,  humidity  of  the  air,  force  and  direction  of  winds, 
elevation  above  sea  level,  and  nature  of  the  soil  are  all  factors 
in  determining  the  boundaries  of  regions  of  vegetation,  there  is 
no  factor  more  important  than  the  annual  rainfall.    Of  course 
the  rainfall  itself  is  largely  determined  by  several  of  the  other 
circumstances  above  mentioned. 

In  the  United  States  this  varies  greatly,  the  yearly  averages 
for  some  of  the  most  important  areas  being  about  as  follows : 

AVERAGE  RAINFALL  PER  YEAR 
REGION  INCHES 

New  England  and  Middle  States 43 

Eastern  Gulf  States 55 

Ohio  basin 44 

Missouri  basin .31 

Rocky  Mountains,  middle  of  eastern  slope     ....     20 

Rocky  Mountains  plateau,  middle 9 

Pacific  slope,  northern  portion 37 

Pacific  slope,  southern  portion 10 


PLANT   GEOGRAPHY  OF  THE   UNITED  STATES     489 

It  is  evident  that  the  rainfall  incr eases  southward  along  the 
Atlantic  coast,  but  that  on  the  Pacific  coast  it  diminishes  south- 
ward. Passing  from  either  coast  inland,  one  finds  the  rainfall 
diminishing  until  it  reaches  a  minimum  in  the  Rocky  Mountain 
region. 

467.  Plant  geography  of  the  United  States.  All  of  the  con- 
tinuous territory  of  the  United  States1  lies  in  the  north  tem- 
perate zone.  There  is  material  for  volumes  in  the  discussion  of 


FIG.  373.   Annual  rainfall  of  the  United  States 

Darkest  shade,  over  80  inches ;  lighter  vertical  lines,  from  40  inches  to  80  inches  ; 
horizontal  lines,  from  20  inches  to  40  inches ;  blank,  from  10  inches  to  20  inches ; 
dotted,  less  than  10  inches.  —  After  W.  M.  Davis 

the  distribution  of  plants  over  our  territory  in  this  continent 
alone,  but  it  is  possible  to  sum  up  a  brief  outline  of  the  matter 
fcin  a  few  pages.  Excluding  the  floras  of  many  single  mountains 
and  mountain  ranges,  the  land  surface  of  the  country  may  for 
botanical  purposes  be  divided  into  four  great  areas,  as  follows : 
The  forest  region.  This  occupies  the  eastern  and  central 
portion  of  the  United  States.  It  is  bounded  on  the  west  by  an 


1  That  is,  not  counting  in  Alaska,  our  West  Indian   possessions,   the 
Hawaiian  Islands,  or  the  Philippines. 


490  PLANT  GEOGRAPHY 

irregular  Hue,  most  of  which  is  east  of  the  hundredth  meridian. 
In  some  places  this  forest  boundary  lies  considerably  east  of 
the  Mississippi  River,  while  in  others  it  extends  from  the  river 
five  hundred  miles  or  more  to  the  westward. 

The  plains  region.  This  stretches  westward  from  the  region 
above-named  to  the  Rocky  Mountain  plateau. 

The  Rocky  Mountain  region.  This  includes  the  Rocky  Moun- 
tains, the  Sierra  Nevada,  and  the  various  plateaus  between 
them. 

The  Pacific  slope.  This  extends  from  the  Cascade  Range  and 
the  Sierra  Nevada  to  the  sea. 

468.  The  forest  region.  The  forest  region  is  mainly  remark- 
able for  its  great  variety  of  hardwood  trees,  of  which  it  contains 
a  larger  number  of  useful  species  than  any  equal  area  of  the 
earth  with  a  temperate  climate.  Perhaps  the  most  important  of 
these  are  the  oaks ;  but  other  genera,  such  as  the  hickory,  the 
tulip  tree,  and  the  sassafras,  are  more  characteristically  Ameri- 
can. In  the  northeasterly  portion  there  are  extensive  forests  of 
the  cone-bearing  evergreens,  such  as  pines,  spruces,  hemlocks, 
and  cedars ;  the  other  trees  which  accompany  these  are  mostly 
deciduous  hardwood  species.  In  the  southerly  portion  the  for- 
ests are  partly  of  coniferous  evergreens  (Fig.  392)  and  partly  of 
deciduous  mesophytes,  such  as  hickories,  beeches,  oaks,  elms, 
hackberries,  magnolias,  and  sycamores.  There  is  also  a  consider- 
able admixture  of  such  hydrophytes  as  the  water  hickory,  the 
sweet  bay  (Magnolia),  the  anise  tree  (Illicium),  the  custard  apple 
(Anona),  the  red  bay  (Persea),  the  loblolly  bay  (Gordonia),  and 
the  sour  gum  (Nyssa),  due  to  the  mild,  moist  climate. 

This  region  was  never  completely  forest-covered.  Areas  of 
prairie,  so-called  "openings"  in  the  hardwood  forests  (Fig.  393), 
extensive  marshes,  and  some  heaths  have  for  ages  been  treeless, 
or  nearly  so.  Generally,  in  the  older  states,  the  land  most  desir- 
able for  cultivation  has  been  tilled  so  long  that  it  is  difficult  to 
find  portions  in  anything  like  their  primitive  condition.  It  is 
only  in  broken  country  like  that  of  the  mountainous  regions 


492  PLANT  GEOGRAPHY 

of  eastern  Tennessee  and  North  Carolina,  the  Adirondack s  and 
the  White  Mountains,  in  swampy  river  valleys,  in  a  few  great 
marshes,  or  in  sterile,  sandy  pine  barrens,  that  one  can  find  the 
original  flora  in  its  natural  condition. 

Comparing  our  forest  region  with  the  parts  of  Europe  which 
resemble  it  most  in  soil  and  climate,  our  flora  differs  notably  in 
possessing  such  leguminous  trees  as  the  locust  and  the  honey 
locusts,  in  the  abundance  of  members  of  the  heath  family,  and 
in  wealth  of  Composites,  especially  asters  and  golden-rods. 

In  very  many  instances  our  eastern  flora  when  it  differs  most 
notably  from  that  of  Europe  greatly  resembles  that  of  China  and 
Japan.  This  is  undoubtedly  due  to  the  fact  that  these  American 
species  and  kindred  Chinese  and  Japanese  ones  had  in  an  earlier 
geological  age  a  common  ancestry. 

On  account  of  the  great  length  of  the  territory  along  a  north 
and  south  axis  and  the  diversified  nature  of  its  surface,  the  flora 
of  the  forest  region  varies  from  a  sub-tropical  one  in  southern 
Florida  to  one  with  a  plentiful  sprinkling  of  sub-arctic  species 
along  portions  of  the  northern  border,  particularly  on  the  higher 
mountains. 

469.  The  plains  region.  This  region  rises  with  a  gradual 
ascent  from  the  prairies  (some  of  which  occur  from  Ohio 
westward  and  over  great  areas  border  the  west  bank  of  the 
Mississippi),  until  an  elevation  of  five  thousand  feet  or  more  is 
attained,  when  the  plains  reach  the  Rocky  Mountain  system. 
There  is  no  sharply  defined  line  of  demarcation  between  the 
prairies  of  western  Kansas,  western  Iowa,  Minnesota,  Nebraska, 
and  South  Dakota,  with  less  than  20  per  cent  of  the  surface 
wooded,  and  the  high  plains,  wholly  treeless  except  along  the 
streams  (Plate  IX,  upper  figure),  that  flank  the  eastern  border 
of  the  Eocky  Mountains.  The  lack  of  trees  in  the  prairie  and 
plains  region  has  been  attributed  to  various  causes,  but  the  prin- 
cipal ones  are  doubtless  forest  fires,  the  scanty  rainfall,  and  the 
occurrence  in  winter  of  severe  drying  winds,  at  a  time  when 
the  roots  can  draw  no  moisture  from  the  frozen  soil. 


THE  ROCKY  MOUNTAIN    REGION  493 

The  vegetation  of  the  prairies  consists  primarily  of  a  con- 
siderable number  of  vigorous  sod-forming  grasses  intermixed 
with  many  other  seed  plants.  Notable  among  these  are  several 
species  of  the  pea  family,  many  golden-rods  and  asters,  and 
some  larger  Composites,  such  as  sunflowers  and  rosinweeds 
(Silphium).  Especially  striking  is  the  display  in  late  summer 
and  autumn  of  many  showy  Composites,  such  as  the  blazing 
star  (Liatris),  the  cone  flower  (Rudbeckia),  and  the  tickseed 
(Coreopsis). 

The  vegetation  of  the  high  treeless  plains  is,  in  the  eastern 
portion  (Plate  IX,  lower  figure),  characterized  mainly  by  the  close 
mats  of  the  short,  xerophytic  buffalo  grasses  and  grama  grasses 
of  a  grayish-green  color.  Among  these  grasses  are  scattered 
prickly  pear  cactuses  (Opuntia),  milkweeds  (Asclepias),  and 
thistles.  After  the  drying  up  of  the  grasses  in  early  July,  there 
is  sometimes  hardly  any  living  vegetation  left  above  ground 
except  that  of  the  cactuses. 

Toward  the  Rocky  Mountains,  as  the  soil  becomes  more  alka- 
line, various  species  of  wormwood  or  sagebrush,  and  members 
of  the  pigweed  family  (Chenopodiacece)  become  predominant. 
The  universal  sagebrush  (Artemisia  tridentata)  plainly  shows 
its  xerophytic  character  by  its  deep-reaching  roots,  its  reduced 
leaf  area,  and  its  strongly  hairy  surface. 

470.  The  Rocky  Mountain  region.  The  Rocky  Mountain 
region  includes  a  very  great-  variety  of  plant  formations,  from 
the  heavily  wooded  mountain  slopes  and  valleys  to  high  sterile 
plains  which  are  almost  deserts.  Cone-bearing  evergreen  trees, 
especially  the  true  spruces,  the  "Douglas  spruce"  (Pseudotsuga), 
and  several  pines,  are  very  characteristic  of  the  forests  (Plate 
XII).  Great  numbers  of  alpine  species  of  herbs  and  shrubs  are 
found  on  the  mountains  at  and  above  the  timber  line.  In  the 
"  alkali "  regions,  where  the  soil  is  too  full  of  mineral  salts  to 
permit  ordinary  plants  to  grow,  many  kinds  of  halophytes,  such 
as  the  salty  sage  (A triplex),  the  greasewood  (Sarcobatus),  Sali- 
cornia,  and  Suceda,  occur. 


494  PLANT  GEOGRAPHY 

Most  notable  among  the  saline  areas  is  the  Great  Basin,  west 
of  the  Great  Salt  Lake,  a  dreary  region  in  general,  destitute  of 
natural  grass  lands  or  trees,  but  with  a  scattered  vegetation  of 
low  gray  or  dull  green  shrubs  and  herbs.  In  the  lower  highly 
alkaline  valleys  are  found  such  halophytic  species  as  those 
above-named,  while  the  drier  valleys  and  foothills  are  somewhat 
evenly  covered  with  sagebrush. 

In  the  South,  cactuses,  palms,  and  tree  yuccas  abound.  Wher- 
ever the  soil  is  gravelly  throughout  the  southern  arid  region,  up 
to  an  elevation  of  five  thousand  feet  or  somewhat  more,  the 
creosote  bush  (Larrea  tridentata)  is  often  as  exclusive  in  its 
occupancy  of  the  ground  as  the  sagebrush  is  in  the  central 
and  northern  parts  of  the  Great  Basin. 

Here  are  some  of  the  most  notable  arid  regions  of  the  United 
States,  such  as  the  Mohave  Desert,  the  Ealston  Desert,  and  the 
Colorado  Desert  of  southern  California.  The  intense  dryness  of 
such  areas  may  be  understood  from  the  fact  that  the  average 
rainfall  of  ten  of  these  deserts  is  only  five  inches  a  year,  and  the 
temperature  in  one  of  them  (at  Fort  Yuma,  Arizona)  remains 
for  weeks  as  high  as  118°  during  the  day,  with  sometimes  only 
a  little  over  one  half  inch  of  rain  a  year. 

471.  The  Pacific  slope.  The  Pacific  coast  region  offers  far 
less  marked  contrasts  between  the  summer  and  winter  temper- 
ature than  are  found  along  the  Atlantic  coast. 

On  the  other  hand,  there  is,  in  the  southern  portion  of  the 
region,  a  sharply  defined  division  of  the  year  into  a  dry  and  a 
rainy  season.  At  San  Diego  the  dry  season  begins  with  April 
and  lasts  for  seven  months.  The  development  of  vegetation, 
therefore,  as  in  the  northerly  part  of  the  plains  region  east  of  the 
Rocky  Mountains,  is  most  rapid  in  spring  and  largely  ceases 
when  the  soil  has  become  parched  by  the  summer's  heat. 

The  flora  of  the  Pacific  slope  is  best  known  by  its  extraordi- 
nary coniferous  evergreen  trees.  In  the  moss-carpeted  woods  of 
the  northern  portion  (bounded  on  the  south  by  the  forty-first  par- 
allel) are  found  the  Port  Orford  cedar  (Cupressus  Lawsoniana), 


PLATE  XII.   A  coniferous  forest  in  central  Colorado,  Douglas  spruce 

(Pseudotsuga  mucronata) 

After  F.  E.  Clements 


THE  PACIFIC   SLOPE  495 

the  red  cedar,  the  tide-land  spruce  (Picea  sitchensis),  and  the 
hemlock  spruce  (Tsuga  lieterophylla).  In  places  there  occur 
dense  thickets  of  hazel  and  inaple,  or  of  shrubs  of  the  heath 
family. 

In  the  southern  portion  of  the  Pacific  slope  (from  the  forty- 
first  to  the  thirty-fifth  parallel)  are  found  the  well-known  Cali- 
fornia evergreen  conifers,  such  as  the  sugar  pine  (P.  Lambertiana) 
of  the  coast,  the  yellow  pine  (P.  ponder  osa),  and  in  the  moun- 
tains the  smaller  redwood  (Sequoia  sempervirens)  and  the  giant 
redwood  (S.  gigantea,  Fig.  33),  the  largest  and  by  far  the  most 
monumental  of  trees. 

Among  the  characteristic  features  of  the  California  flora  is 
the  abundance  of  xerophytic  shrubs  and  small  trees,  many  of 
them  broad-leaved  (not  coniferous)  evergreens,  forming  the 
chaparral  thickets.  Among  these  are  members  of  the  oak,  the 
rose,  the  sumach,  the  heath,  the  buckthorn,  the  composite  family, 
and  many  others. 

In  southern  California,  on  account  of  the  long  dry  season, 
plants  with  large  roots  or  rootstocks,  and  bulb-bearing  plants, 
many  of  them  belonging  to  the  lily  family,  are  abundant. 

In  the  deserts  and  on  their  borders  are  numerous  cactuses 
and  other  succulent  forms.  Among  the  most  characteristic 
desert  plants  are  the  Spanish  bayonets,  or  Yuccas,  some  of 
them  tree-like  in  form  and  size. 


CHAPTEE  XXXIX 
VARIATION,   MUTATION,  AND  ORIGIN  OF  SPECIES 

472.  Variations  of  plants.    One  of  the  foundation  principles 
of  scientific  farming  and  gardening  is  that  seeds  will  grow  into 
plants  like  those  which  produced  them.    Not  only  is  it  assumed 
that  grains  of  corn  will  grow  into  corn  plants  and  beans  into 
bean  plants,  but  also  that  any  special  variety  of  sweet  corn  will 
produce  its  like,  yellow-eyed  beans  their  like,  and  so  with  mul- 
titudes of  familiar  cases.    Closer  observation,  however,  shows 
that  no  two  of  the  hundreds  or  thousands  of  plants  raised  from 
the  seeds  of  a  single  parent  plant  will  be  exactly  like  each  other 
or  the  parent.    Generally  the  variations  are  very  slight,  and  most 
of  them  fail  to  continue  themselves  in  succeeding  generations 
so  as  to  establish  new  varieties  of  plants. 

473.  Variations  in  one  direction.    While  variation  generally 
goes  on  in  all  directions,  so  that  one  of  a  brood  sprung  from  a 
given  parent  will  be  smaller  and  another  larger,  one  more  and 
another  less  hairy  than  the  parent  plant,  and  so  on,  it  is  not 
uncommon  to  find  what  may  be  called  definite  variation,  in 
which  the  changes  all  lead  toward  a  definite  new  type.    The 
behavior  of  lowland  forms  planted  in  alpine  regions  (Sec.  464) 
is  a  good  instance  of  the  kind.    It  is  well  known,  too,  that  seed 
from  northern  localities  when  planted  farther  south  will  produce 
earlier  crops  than  can  be  obtained  from  southern  seed.   American 
varieties  of  onion,  after  being  grown  for  a  series  of  years  in  Eng- 
land, become  habituated  to  the  longer  mild  season  there,  and 
when  the  seed  is  brought  back  to  America  the  plants  grown  from 
it  fail  to  mature  their  bulbs  before  the  coming  of  the  frost. 

Such  facts  as  these  seem  to  indicate  that  characteristics  which 
have  been  impressed  upon  the  plant  by  external  influences,  such 

496 


IMPORTANCE  OF  ADAPTIVENESS  IN   PLANTS       497 

as  those  of  soil  and  climate,  may  be  transmitted  to  its  descend- 
ants. If  it  be  so,  then  the  origination  of  new  forms  of  plants 
by  the  inheritance  of  such  characteristics  must  be  extremely 
common. 

474.  Mutations  of  plants.    Much  attention  has  lately  been 
given  to  the  occurrence  among  plants  of  seedlings  which  differ 
in  a  marked  way  from  the  parents.    It  would  involve  too  much 
detail  to  describe  the  exact  nature  of  the  differences  between 
the  seedlings  of  the  evening  primrose,1  which  has  been  most 
studied  in  this  connection,  and  its  offspring,  but  they  are  as 
great  as  those  between  an  apple  tree  and  a  pear  tree.    Such 
abrupt  and  extensive  changes  are  called  mutations.    A  few  of 
the  most  important  facts  so  far  known  in  regard  to  mutation  are  : 

1.  New  species  2  appear  suddenly  among  the  offspring  of  the 
parent  form. 

2.  The  individuals  of  the  new  species  constitute  only  a  small 
per  cent  of  any  given  brood. 

3.  The  new  species  reproduce  themselves  accurately,  showing 
no  decided  tendency  to  return  to  the  parent  form. 

475.  Importance  of  adaptiveness  in  plants.    It  may  be  in- 
ferred from  Chapters  xxxi  and  xxxiv  that  a  premium  is  set  on  all 
changes  in  structure  or  habits  which  may  enable  plants  to  resist 
their  living  enemies  or  to  live  amid  partially  adverse  surround- 
ings of  soil  or  climate.    It  would  take  a  volume  to  state,  even  in 
a  very  simple  way,  the  conclusions  which  naturalists  have  drawn 
from  this  fact  of  a  savage  competition  going  on  among  living 
things,  and  it  will  be  enough  to  say  here  that  the  existing  kinds 
of  plants  to  a  great  degree  owe  their  structure  and  habits  to  the 
operation  of  the  struggle  for  existence,  together  with  their  response 
~by  means  of  variation  to  changes  in  the  conditions  ~by  which  they 
are  surrounded.    How  the  struggle  for  existence  has  brought 
about  such  far-reaching  results  will  be  briefly  indicated  in  the 
next  section. 

1  (Enothera  lamarckiana. 

2  For  a  definition  of  the  term  species,  see  Sec.  189. 


498      VARIATION,  MUTATION,  AND   ORIGIN   OF  SPECIES 

476.  Survival  of  the  fittest.    A  change  in  the  characteristics 
of  a  species  may  have  no  effect  on  its  ability  to  contend  with 
a  hostile  soil  or  climate,  with  parasitic  plants  or  destructive 
insects  or  other  animal  foes ;  but  often  alterations  in  the  struc- 
ture or  the  habits  of  a  plant  may  give  it  a  considerable  advan- 
tage over  its  unchanged  neighbors.    For  instance,  a  decided  in- 
crease in  hairiness  would  tend  to  protect  the  plant  from  damage 
by  long  droughts,  and  also  (in  countries  where  snails  destroy 
much  vegetation)  from  having  its    leaves    eaten.    Nuts    with 
harder  shells  would  escape  being  destroyed  when  the  ordinary 
ones  would  be  cracked  and  eaten  by  wild  animals.    Eed  berries 
of  the  European  holly  are  carried  off  by  birds  more  extensively 
than  yellow  ones,  and  thus  the  undigested  seeds  of  the  former 
variety  are  more  widely  sown. 

In  meadows  which  are  mown  once  a  year,  only  those  plants 
can  surely  reproduce  themselves  by  seed  which  ripen  their  seeds 
either  before  or  after  the  time  when  the  grass  is  cut.  Individ- 
uals which  can  do  this  stand  a  vastly  greater  chance  of  perpetu- 
ating themselves  than  do  those  which  are  cut  down  just  before 
their  seeds  have  matured.  For  this  reason  certain  kinds  of 
meadow-frequenting  plants l  have  developed  early-blooming  and 
late-blooming  forms,  which  would  probably  never  have  become 
abundant  in  regions  where  the  grass  was  not  mown. 

Whatever  the  nature  of  the  advantages  given  to  one  form  or 
set  of  forms  over  another  in  the  competition  which  always  goes 
on  under  natural  conditions,  it  results  in  what  is  sometimes 
called  survival  of  the  fittest,  and  sometimes  natural  selection. 

477.  Have  species  arisen  by  variation  or  by  mutation  ?   The 
theory  that  species  (and  later  genera  and  higher  groups)  arise 
by  slow  degrees  from  the  operation  of  natural  selection  acting 
on  the  slight  variations  which  constantly  occur  among  animals 
and  plants  was  first  fully  set  forth  by  Charles  Darwin  in  1858.2 

1  Species  of  Gentiana,  Euphrasia,  and  Rhinanthus. 

2  Darwin's  paper  on  this  subject  was  the  result  of  over  twenty  years  of 
study,  and  was  read  by  him  to  accompany  a  paper  containing  similar  views 
which  had  been  sent  from  the  East  Indies  by  Alfred  Russel  Wallace. 


HOW   SPECIES  HAVE  ARISEN  499 

The  theory  that  species  spring  suddenly  from  mutations  was 
advanced  by  Professor  Hugo  de  Vries,  of  Amsterdam,  Holland, 
in  a  work  on  the  mutation  theory,  published  in  1901  and  1903. 

Botanists  at  present  are  considerably  divided  on  the  question 
of  the  origin  of  species,  some  believing  that  they  are  mainly 
derived  from  the  perpetuation  and  intensification  of  slight  varia- 
tions, while  mutations  are  so  infrequent  as  not  to  signify  much 
in  this  connection ;  others,  again,  believe  that  mutations  are 
the  source  of  species,  and  that  variations  can  only  give  rise  to 
varieties.  There  seems  to  be  no  good  reason  for  doubting  that 
both  variation  and  mutation  have  been  and  are  efficient  in  the 
production  of  new  species. 


CHAPTER  XL 
PLANT  BREEDING 

478.  Definition  of  plant  breeding.    The  selection  and  mainte- 
nance of  the  most  desirable  varieties  of  cultivated  plants  must, 
to  some  extent,  have  occupied  the  attention  of  agriculturists 
during  all  the  thousands  of  years  since  farming  began.1    From 
the  writings  of  Virgil  and  other  Latin  authors  it  is  clear  that 
Roman  farmers  practiced  careful  selection  of  cereals  for  seed, 
knowing  that  without  this  their  crops  would  diminish.    But  it 
is  only  within  a  short  period  that  scientific  principles  have  been 
brought  to  bear  on  the  process.    In  fact,  it  is  stated  that  the 
systematic  improvement  of  races  of  cultivated  plants  began  in 
the  middle  of  the  nineteenth  century.    The  intentional  produc- 
tion and    perpetuation  of   new  varieties    is    known    as  plant 
breeding.    It  is  based  upon  the  methods  outlined  in  Sees.  479 
and  480. 

479.  Single  selection  and  continued  selection.     New  varie- 
ties of  plants,  whether  wild  or  cultivated,  are  constantly  being 
produced  by  ordinary  variation  and  by  mutation  (Sees.  472-474). 
In  a  single  field,  supposed  to  contain  only  one  kind  of  wheat,  a 
trained  botanist  once  found  twenty-three  well-marked  varieties, 
one  of  which  became  the  parent  of  a  sort  that  has  remained  famous 
for  over  three  quarters  of  a  century.    The  plant  breeder  is  con- 
stantly on  the  watch  for  promising  varieties,  preserving  all  which 
seem  likely  to  be  of  use.    While  it  is  a  slow,  uncertain  method 
to  await  the  appearance  of  variations  in  any  desired  direction, 
and  then  to  rely  on  the  perpetuation  of  these,  the  large  num- 
ber of  valuable  new  varieties  thus  secured  warrants  all  growers 

1  In  China  the  cultivation  of  rice,  wheat,  two  kinds  of  millet,  and  soy 
beans  dates  back  at  least  4600  years. 

500 


PRODUCTION  OF  HYBRIDS  501 

in  being  on  the  lookout  for  variations  which  promise  new 
values.  The  surer  plan  is  to  take  seed  from  a  considerable 
number  of  parent  individuals  which  possess  the  desired  quality 
in  a  high  degree,  raise  plants  from  each  of  these,  discard  plants 
of  this  second  generation  from  all  parents  whose  progeny  does 
not  excel,  and  continue  selecting  from  these  superior  stocks. 
In  this  way  many  characteristics,  such  as  abundant  yield,  hardi- 
ness, early  ripening,  whiteness  in  the  case  of  flour,  increased 
percentage  of  sugar  in  sugar  beets,  or  improved  size  or  flavor  in 
tomatoes  may  be  secured  in  a  few  years  of  careful  breeding. 
This  may  be  called  the  selection  of  good  parent  plants. 

480.  Production  of  hybrids.  An  important  method  of  mak- 
ing new  varieties  is  by  crossing,  or  hybridizing,  —  that  is,  by 
pollinating  the  pistil  of  one  species  or  variety  with  pollen  from 
another  species  or  variety.  The  offspring  of  cross  pollination  is 
known  as  a  hybrid. 

The  process  of  crossing  two  species  is  comparatively  easy.  If 
plum  blossoms,  for  example,  are  to  be  hybridized,  the  operator 
must  gather  enough  of  those  from  which  pollen  is  to  be  col- 
lected, brush  or  shake  off  the  pollen,  and,  if  necessary,  keep  it 
in  a  cool  place  until  needed.1  Most  of  the  flower  buds  are 
removed  from  the  tree  the  flowers  of  which  are  to  be  pollinated, 
and  just  before  the  opening  of  those  buds  which  are  left  the 
corolla,  with  its  attached  anthers,  is  cut  away,  as  shown  in 
Fig.  375,  and  pollen  applied  to  the  stigmas  with  a  camel's-hair 
pencil  or,  better,  with  the  finger  tip.  If  fertilization  results, 
and  plums  with  good  seeds  are  produced,  they  must  be  planted, 
and  seedling  trees  grown  from  them.  These  might  be  allowed 
to  grow  until  they  blossomed,  but  years  of  valuable  time  can 
be  saved  by  grafting  the  young  seedlings  upon  other  plum  trees. 
When  blossoms  of  the  hybrid  form  are  secured,  some  of  them 
may  be  fertilized  with  pollen  of  either  of  the  parent  species, 

1  Some  kinds  of  pollen,  as  that  of  the  pansy  and  the  peony,  are  said  to 
remain  good  for  weeks,  and  that  of  the  date  palm  for  more  than  a  year ;  but 
in  general,  pollen  should  be  used  as  soon  as  possible. 


502 


PLANT  BREEDING 


and  others  with  pollen  of  different  species  of  plum.  All  of  the 
seeds  obtained  from  the  various  crosses  should  be  planted,  and  the 
seedlings  which  are  produced  by  them  should  be  examined,  and 
retained  or  destroyed  according  to  their  apparent  value.  To  the 
experienced  plant  breeder  the  appearance  of  the  seedling  trees, 
long  before  they  are  old  enough  to  blossom,  indicates  so  much  as 

to  the  nature  of  their  fruit  that  many 
varieties  can  be  discarded  as  soon 
as  the  young  plants  have  developed 
well-grown  shoots.  The  distinctive 
work  of  hybridizing  is  to  secure 
parent  plants  better  than  any  which 
exist  in  the  foundation  species  or 
varieties.  The  work  of  choosing  a 
large  number  of  the  most  promising 
hybrid  plants  and  of  testing  their 

FIG.  375.   A  plum  blossom  pre-    breeding  power,  so  that   only  the 
pared  for  hybridizing  |)loodj  go  to  speak?  of  tlie  yery  best 

A,  unopened  blossom  cut  round  may  be  retained,  is  the  same  as 

just  below  the  insertion  of  the    ,  , .          ,  ,        .  .          n 

stamens,  to  remove  the  latter;   breeding  by  selection  mentioned 
B,  lengthwise  section  of  a  fully  above.    In  the  occasional   hybrid 

opened    blossom,    showing    the      ,  .,  , 

level  s  at  which   the  cutting  plant,  possibly  one  out  of  ten  thou- 
should  be  done  sand,  are  combined  the  best  in  the 

two  parents,  or  possibly,  as  some  believe,  newly  created  char- 
acters may  arise. 

481.  Some  results  in  breeding   by  selection.    To  give  an 
account  of  the  results  of  selection  as  applied  to  cultivated  plants 
would  be  to  write  a  history  of  the  variations  and  improvements 
in  all  our  ornamental  and  useful  plants  under  cultivation.    In 
this  place  it  must  suffice  to  give  a  very  few  illustrations  of  the 
kind  and  amount  of  improvement  brought  about  by  such  selec- 
tion as  is  outlined  in  Sec.  479. 

482.  Selection  among    apples.    Much   of  the  improvement 
in  apples  was  brought  about  before  the  literature  of  plant  breed- 
ing began.    It  is  not  certainly  known  where  the  cultivated  apple 


SELECTION  AMONG  APPLES 


503 


originated,  but  an  eatable  variety  probably  occurred  in  prehis- 
toric times  throughout  the  territory  extending  from  the  Caspian 
Sea  nearly  to  Europe.  Small  forests  of  wild  apples  have  been 
described  in  modern  times,  growing  near  the  southeast  end  of 
the  Black  Sea. 

The  dwellers  in  pile-built  houses  in  the  lakes  of  northern 
Italy,  Savoy,  and  Switzerland,  several  thousands  of  years  ago, 
laid  in  stocks  of  apples  cut  and  dried  for  whiter  use.  Some 
of  these  apples  appear  to 
have  been  cultivated,  but 
they  were  very  small,— 
inferior  in  size  to  any  mod- 
ern variety  except  some 
crab  apples.  How  great  a 
gain  in  the  size  of  apples 
has  been  brought  about  by 
cultivation  and  selection 
may  be  inferred  from  Fig. 
376.  This  increase  in  bulk 
is  accompanied  by  a  de- 
crease in  the  number  of 
matured  fruits  in  a  cluster.  Fie-  376-  Mect  "f  cultivation  upon  the 

size  of  apples 


The  Bismarck  apple,  with  a,  the  wild  Asiatic 
crab  apple  (Pyrus  baccata),  and  b,  the  Eu- 
ropean wild  apple  (P.  mains).  All  half  nat- 
ural size.  —  After  Hodge 


Originally  several  of  the 
flowers  developed  into  ap- 
ples, but  in  modern  im- 
proved varieties  usually 
all  but  one  of  the  flowers  fail,  as  is  shown  in  the  case  of  the 
pear  (Fig.  83). 

Most  of  the  varieties  of  apples  in  our  present  orchards  are 
descendants  of  seedlings  sprung  from  trees  introduced  from 
western  Europe.  In  the  Northwest,  where  only  the  hardiest 
kinds  can  endure  the  severe  climate,  some  of  the  most  success- 
ful sorts  are  importations  from  central  Eussia,  and  others  are 
from  seedlings  of  Eussian  and  the  hardier  American  varieties,  or 
from  hybrids  produced. by  accident  or  design. 


504  PLANT   BREEDING 

483.  Selection  among  beans.    The  common  bean  (Phaseolus 
vulgaris)  is  of  uncertain  origin,  but  there  is  a  good  deal  of  evi- 
dence to  show  that  it  came  from  western  South  America.    Its 
cultivation  in  Europe  appears  to  have  begun  soon  after  the  dis- 
covery of  America.    As  is  well  known,  the  number  of  varieties 
in  cultivation  is  very  large,  and  in  few  plants  is  it  easier  than  in 
beans  to  produce  new  varieties  by  selection. 

Bean  breeding  for  the  large  seedsmen  is  a  skilled  industry. 
It  is  said  that  a  seedsman  may  even  advertise  a  new  kind  of 
bean  under  an  attractive  name  before  the  variety  has  been  pro- 
duced, then  order  it  of  his  bean  grower,  and  in  the  course  of 
two  or  three  years  have  seed  ready  for  his  customers.  On  the 
farm  of  one  large  bean  grower  nearly  70  standard  varieties  are 
raised  for  seed  on  a  large  scale,  and  some  200  sorts  are  being 
tested  to  establish  their  value  or  to  produce  new  kinds.  All  pos- 
sible pains  are  taken  by  means  of  high  cultivation  to  increase  the 
bearing  qualities  of  the  plants  and  also  to  encourage  variation. 
Every  variety,  whether  a  standard  one  or  a  novelty,  is  kept  to 
the  desired  type  by  the  careful  inspection  of  every  plant,  those 
which  fall  short  in  any  respect  being  carefully  destroyed. 

While  new  kinds  are  nowadays  generally  secured  by  scien- 
tific plant  breeding,  sometimes  valuable  sorts  are  obtained  from 
chance  seedlings,  as  in  the  case  of  a  well-known  dwarf  Lima 
bean  which  sprang  from  seeds  gathered  on  a  Virginia  roadside 
some  time  before  1885. 

484.  Selection  among  corn.    Indian  corn  was  cultivated  by 
the  ancient  Peruvians  and  the  Mexicans.    Its  original  home  as 
a  wild  plant  was  probably  on  or  near  the  west  coast  of  South 
or  Central  America.    Numerous  rather  permanent  kinds  which 
"  come  true  from  the  seed "   (races),  such  as  field  corn,  sweet 
corn,  and  pop  corn,  have  long  been  known,  and  some  of  these 
races  present  many  varieties. 

Scientific  corn  breeding  has  been  practiced  for  much  less  than 
a  generation,  but  the  results  already  attained  are  of  great 
practical  importance. 


SELECTION   AMONG  CORN 


505 


Leaving  out  of  account  the  very  extensive  use  of  the  stems  and 
leaves  of  the  corn  plant  for  forage,  and  considering  only  the  value 
of  the  grain  produced,  corn  breeding  may  be  carried  on  to  secure, 
among  other  less  important  qualities,  the  following  results : 

1.  A  larger  yield  per  acre. 

2.  A  higher  percentage  of  any  one  of  the  three  principal  constitu- 
ents of  the  grain,  —  starch,  proteids,  and  oil. 

3.  Early  maturing,  for  growth  in  the  more  northerly  states. 


A  B 

FIG.  377.    Kernels  of  corn  with  high  and  with  low  proteid  contents 

A,  high  proteids;  B,  low  proteids;  />,  horny  layer,  consisting  largely  of  proteids; 
s,  white  starchy  portion;  e,  embryo.  —  After  University  of  Illinois  Agricul- 
tural Experiment  Station,  Bulletin  No.  87 

1.   Yield.   The  corn  crop  of  the  United  States  is  worth  about 
a  billion  dollars  a  year  for  the  grain  alone.    On  farms  of  the 


FIG.  378.    Kernels  of  corn  with  high  and  with  low  oil  contents 

A,  Ai,  cross  and  lengthwise  section  of  high  oil  kernels;  B,  B\,  sections  of  low 
oil  kernels;  e,  embryo.  Most  of  the  oil  is  contained  in  the  embryo,  so  that 
a  large  embryo  means  a  large  percentage  of  oil.  —  After  University  of  Illi- 
nois Agricultural  Experiment  Station,  Bulletin  No.  87 


506  PLANT  BREEDING 

greatest  producing  state,  Illinois,  the  average  crop  is  hardly 
thirty  bushels  per  acre.  The  use  of  choice  seed  has  been  found 
to  increase  the  production  from  10  to  20  per  cent,  and  it  is  a 
moderate  estimate  which  assumes  that  the  universal  use  of 
improved  seed  would  add  10  per  cent  to  the  total  corn  crop  of 
the  country.  This  would  add  over  $100,000,000  to  the  annual 
receipts  of  our  corn  growers. 

2.  Improved  quality.    In  every  100  pounds  of  ordinary  shelled 
corn  there  are,  in  round  numbers,  about 

8  lb.  embryo  (of  which  3  Ib.  are  oil) ; 

13  lb.  gluten,  or  proteids  of  the  endosperm ; 

64  lb.  starch. 

There  is  a  demand  for  a  limited  amount  of  corn  with  a  high 
per  cent  of  oil  as  a  source  of  com  oil.  At  the  Illinois  Agricul- 
tural Experiment  Station  the  attempt  has  been  made  to  breed 
varieties  of  corn  with  high  and  with  low  percentages  of  oil. 
One  variety  was  secured  with  nearly  7  per  cent  and  another 
with  less  than  2  per  cent. 

In  the  same  way,  that  is  by  means  of  continued  selection, 
carried  through  many  generations,  varieties  with  much  or  little 
starch  can  be  obtained. 

3.  Early  maturing.    Corn  was  originally  a  tropical  or  sub- 
tropical plant,  requiring  a  long  growing  season.    Quickly  matur- 
ing varieties  had,  however,  been  secured  by  the  native  races 
even  at  the  time  of  the  discovery  of  America  by  Columbus.    At 
present  there  are  varieties  ranging  all  the  way  from  the  eighteen- 
foot  kinds  that  require  a  growing  period  of  six  months,  to  the 
two-  or  three-foot  kinds  that  mature  in  ninety  days  or  less. 

The  most  important  problem  that  presents  itself  to  the  plant 
breeder  in  this  connection  is  that  of  increasing  the  yield  per 
acre  for  each  of  the  agricultural  regions  where  corn  is  produced, 
whether  in  the  North,  where  short-stalked,  early-maturing  kinds 
are  needed ;  in  the  great  corn  belt ;  or  in  the  South,  East,  or 
West,  where  varieties  are  needed  which  are  bred  to  make  the 


SELECTION   AMONG  WHEAT  507 

best  yield  of  grain  or  of  fodder,  or  of  grain  and  fodder  combined. 
This  plant  is  being  especially  modified  for  many  agricultural 
regions  possessing  distinctive  soil  and  climatic  conditions,  and 
is  more  easily  adapted  to  locality  than  are  most  plants. 

485.  Selection  among  wheat.  Wheat  of  many  varieties  has 
been  cultivated  for  thousands  of  years  throughout  a  territory 
ranging  all  the  way  from  China  to  western  Europe.  The  origi- 
nal home  of  the  plant  is  not  known,  but  perhaps  it  was  in 
Mesopotamia,  between  the  Tigris  and  Euphrates  rivers.  In 
Europe  systematic  attempts  to  procure  improved  varieties  of 
wheat  by  selection  date  back  well  toward  the  beginning  of  the 
nineteenth  century.  Some  good  varieties  were  originated  in 
our  own  country  in  the  early  sixties,  but  more  wheat  breeding 
is  now  done  in  a  single  year  in  the  Agricultural  Experiment 
Station  of  a  great  wheat-raising  state,  like  Minnesota,  than  was 
done  in  the  whole  United  States  prior  to  1890. 

It  will  give  some  idea  of  the  extreme  care  with  which  wheat 
breeding  is  now  conducted  to  give  the  barest  outline  of  the 
mode  of  procedure  in  the  Minnesota  Station. 

As  a  beginning,  10,000  good  kernels  of  some  desirable 
variety  of  wheat,  old  or  new,  are  carefully  chosen.  These 
grains  are  planted  4  inches  apart  (or  5  inches  for  winter 
wheat),  one  seed  in  a  hill,  and  every  plant  receives  a  number. 
About  95  per  cent  of  the  poorer  plants  are  weeded  out  by 
hand  before  harvesting  the  seed  wheat,  the  heads  of  the  re- 
maining plants  are  cut  off,  and  those  of  each  plant  are  preserved 
in  an  envelope.  After  drying,  the  heads  are  weighed,  and  those 
of  all  but  a  few  of  the  best-yielding  plants  are  thrown  away. 

The  second  season  there  are  sown  in  a  separate  plot  in  the 
wheat-breeding  nursery  about  a  hundred  seeds  from  each  of 
the  plants  chosen.  Each  of  these  hundred-groups  (centgeners), 
sprung  from  a  single  mother  plant,  is  given  a  distinguishing 
number.  When  the  wheat  is  mature*  the  relative  size  and 
strength  of  the  plants  in  each  plot  are  noted  and  recorded, 
and  by  separately  harvesting  and  weighing  each  little  plot  the 


508  PLANT  BREEDING 

breeding  power  of  each  parent  plant  is  measured  in  terms  of 
the  average  of  its  progeny.  A  select  head  is  chosen  from  each 
of  several  of  the  best  plants  in  every  plot,  and  the  seed  from 
these  is  saved. 

A  third  year  and  a  fourth  year  hundred-group  plots  are  sown 
and  managed  as  just  described,  and  at  the  end  of  the  period  the 
most  promising  varieties  are  taken  to  field  trials.  Here  they 
are  tested,  under  ordinary  farm  conditions,  in  comparison  with 
the  wheats  commonly  grown,  and  the  best,  if  it  stands  severe 
milling  tests,  is  then  propagated  for  distribution,  under  suitable 
designating  numbers,  to  wheat  growers  throughout  the  state. 

The  rate  at  which  new  varieties  can  be  propagated  may  be 
gathered  from  the  history  of  one  of  the  most  famous  new 
wheats,  "Minnesota  No.  163,"  a  variety  bred  by  selection.  This 
sprang  from  a  single  grain  planted  in  1892.  In  1893  the 
product  consisted  of  75  plants;  in  1894  a  small  field  plot  was 
grown;  in  1898  the  crop  amounted  to  some  300  bushels  of 
seed  wiieat,  which  was  distributed  among  about  50  farmers 
throughout  the  state.  It  is  estimated  that  in  15  years  from 
the  time  of  planting  the  single  original  seed  the  entire  wheat 
crop  of  Minnesota,  covering  some  5,300,000  acres,  might  have 
been  made  to  consist  of  this  variety,  and  that  it  does  actually 
cover  millions  of  acres,  adding  about  two  dollars  per  acre  to 
the  value  of  the  crop. 

It  is  not  yet  possible  to  state  how  much  can  be  gained  in 
quality  and  quantity  of  wheat  production  by  careful  culture 
and  breeding.  But  it  is  interesting  to  note  that  in  a  good 
wheat  year  (1895),  when  the  average  crop  per  acre  on  the  Uni- 
versity of  Minnesota  farm  was  23  bushels,  there  were  4  im- 
proved varieties  which  yielded  over  40  bushels  per  acre.  In 
1896,  when  the  average  crop  for  the  state  was  14.2  bushels 
per  acre,  out  of  32  improved  varieties  on  the  University  farm 
there  were  24  varieties  which  yielded  21  bushels  per  acre  or 
more,  2  of  them  yielding  33  bushels.  That  is,  three  quarters  of 
the  varieties  yielded  at  least  1^  times  as  much  as  ordinary 


GENERAL  RESULTS  OF  HYBRIDIZATION  509 

wheat  on  other  farms,  and  2  varieties  yielded  about  21  times  as 
much.  The  yield  was  increased  on  the  farm  mentioned  both 
by  good  farm  management  and  by  breeding  into  the  varieties 
stronger  power  of  yielding. 

In  1902  one  of  the  improved  wheats,  "Minnesota  No.  169," 
was  given  an  extended  trial  in  various  parts  of  Minnesota.  It 
yielded  on  the  average  33  bushels  per  acre,  or  18  per  cent  more 
than  the  ordinary  varieties.  This  variety  probably  now  covers 
half  a  million  acres,  in  several  states,  and  yields  at  least  two 
dollars  per  acre  more  value  than  the  varieties  (mainly  the  "  blue 
stem,"  its  parent)  which  it  is  rapidly  displacing  over  an  area  of 
several  million  acres  devoted  to  hard  spring  wheat.  The  impor- 
tance of  every  increase  in  production  is  evident  when  one  con- 
siders the  annual  value  of  our  wheat  crop,  from  $250,000,000 
to  $500,000,000. 

486.  General  results  of  hybridization.  The  relative  impor- 
tance of  hybridization,  and  of  continued  selection  alone  as 
means  of  securing  valuable  new  varieties  of  cultivated  plants, 
is  largely  to  be  worked  out  in  each  class  of  plants.  Plant 
breeding  as  a  science  is  too  new  to  give  material  for  answering 
nearly  all  the  questions  that  naturally  arise  in  regard  to  how 
varieties  may  be  most  rapidly  improved.  Hybridizing  often 
brings  about  great  changes  in  the  offspring,  and  there  are 
increased  chances  that  some  of  the  new  forms  will  be  more 
valuable  than  any  which  could  be  discovered  among  the 
foundation  varieties.  In  the  case  of  species  perpetuated  by 
grafting,  as  of  certain  trees,  and  plants  propagated  by  roots, 
rootstocks,  or  tubers,  as  potatoes,1  it  is  very  easy  to  secure  pure- 
bred stocks.  In  plants  grown  from  seed,  especially  if  the 
species  is  more  or  less  open-pollinated,2  there  is  always  a  most 
important  question  as  to  how  many  generations  must  elapse 
before  the  hybrid  varieties  can  be  selected  "  true  to  seed." 

1  Varieties  among  these  are  called  clonal  varieties  (from  don,  meaning  a 
cutting  or  scion). 

2  That  is,  if  the  flowers  are  open  to  cross  pollination. 


510 


PLANT  BREEDING 


Some  of  the  most  important  results  in  variety  making  by 
hybridization  have  recently  been  obtained  in  experiments  on 
the  fruits  of  the  rose  family,  particularly  cherries,  plums,  and 


FIG.  379.  Five  forms  of  leaf  from  hybrid  blackberries,  all  grown  from  the 
seed  of  one  plant  and  showing  extraordinary  variations  in  the  amount  of 
incision  in  the  margins  of  the  leaflets,  forming  a  regular  series  from  a  to  e 

Modified  after  Burbank 

apples,  and  the  citrous  fruits.    In  the  case  of  cotton  and  wheat 
much  effective  work  is  also  being  done. 

The    extraordinary    successes    of   Luther    Burbank    in    pro- 
ducing new  hybrid  varieties  of  fruits  and  ornamental  flowers 


FIG.  380 

a,  a  stoueless  wild  plum;  6,  c,  d,  fruit  of  hybrids  of  a  with  the  French  prune. 
All  drawn  to  the  same  scale.  —  Modified  after  Burbank 

have  been  widely  discussed  in  the  popular  magazines.  He  has 
bred  some  remarkable  hybrids,  such  as  those  between  the 
strawberry  and  raspberry,  the  apple  and  blackberry,  the  petunia 
and  the  tobacco  plant.  These  are  of  little  use,  though  of  much 
scientific  interest.  Others  of  his  hybrids,  especially  the  plums, 
are  of  great  commercial  value.  Many  other  investigators,  whose 


RESULTS  OF  HYBRIDIZING  CITROUS  FRUITS       511 


results  have  not  received  popular  notice,  are  working  more 
directly  for  useful  hybrids,  and  a  few  *  of  these  may  be  very 
briefly  summed  up. 

487.  Results  of  hybridizing  citrous  fruits.  In  the  plant- 
breeding  laboratory  of  the  United  States  Department  of  Agri- 
culture in  1896  and  1897  hybrids  were  made  of  the  ordinary 
sweet  orange  and  the  uneatable  three-leaved  orange  (Citrus  trifo- 
liata).  Three  promising  varieties  of  a  new  kind  of  fruit  known 
as  citranges  have  thus  been  obtained.  Two  of  these  are  likely 
to  serve  as  substitutes  for 
lemons,  and  the  third  may, 
to  some  extent,  take  the 
place  of  grape  fruit.  Their 
main  value  lies  in  the  fact 
that  they  can  be  cultivated 
from  two  hundred  to  four 
hundred  miles  farther  north 
than  ordinary  citrous  fruits. 

Another  interesting  hy- 
brid is  that  between  the  tan- 
gerine and  the  grape  fruit, 
called  the  tangelo,  which 
shows  a  blending  of  the 
characteristics  of  the  par- 


A  *  B 

FIG.  381.    The  flower  of  the  wheat  plant 


A,  entire  flower  as  seen  at  five  in  the  morn- 
ing, with  the  stamens  protruding,  the  pistil 
remaining  inside ;  13,  the  anther  enlarged, 
showing  escaping  pollen ;  C,  the  pistil  en- 
larged, showing  the  feathery  stigmas. — 


After  University  of  Minnesota  Agricultural 
Experiment  Station 


ent  species. 

488.  Results  of  hybridiz- 
ing cotton.  The  cotton  produced  in  the  United  States  is  roughly 
classed  as  long  staple  and  short  staple.  The  fibers  of  the  former 
kind  are  about  one  and  one-half  times  as  long  as  those  of  the 
latter.  For  many  kinds  of  goods  long  staple  cotton  is  indispen- 
sable, and  its  price  is  from  one  and  one-half  times  to  nearly 
twice  as  great  as  the  price  of  short  staple  cotton.  The  short 
staple  sorts  can  be  grown  over  a  much  larger  territory  than  the 
others,  so  that  our  annual  production  of  long  staple  cotton  is 
only  about  one  and  one-half  per  cent  of  our  total  cotton  crop. 


512 


PLANT  BREEDING 


Hybrids  have  been  made  between  the  very  long-fibered  fine  sea- 
island  species  and  the  ordinary  upland  species,  and  after  six 
generations  of  selection  and  careful  cultivation  some  valuable 
hybrid  varieties  seem  to  have  been  developed. 

489.  Results  of  hybridizing  wheat.  The  flowers  of  wheat 
are  naturally  self-pollinated, —  that  is,  the  stamens  of  each  flower 
commonly  discharge  their  pollen  upon  the  feathery  stigma  of 
their  own  flower  as  soon  as  the  pollen  sacs  open.  This  fact 
makes  hybridization  much  more  effective  in  producing  variation 


FIG.  382.  Variation  in  wheat,  the  hybrid  offspring  of  hybrid  parents 

After  figure  redrawn  from  Transactions  of  the  Highland  and  Agricultural 
Society  of  Scotland 

in  wheat  than  in  plants  which  are  generally  cross-pollinated ; 
for  in  the  case  of  wheat  any  kind  of  cross  pollination,  and 
especially  that  between  markedly  different  varieties,  may  be 
said  to  give  a  sort  of  shock  to  the  operation  of  reproduction, 
and  thus  produce  abundant  variation.  The  details  of  the 
process  of  artificial  pollination  need  not  be  given.  It  is  suc- 
cessful in  a  large  proportion  of  cases,  and  the  offspring  may  be 
of  many  types,  as  shown  by  Fig.  382.  It  is  found  that  after 
the  fourth  generation  an  occasional  plant  may  be  found  which 


RESULTS  OF  HYBRIDIZING  WHEAT  513 

yields  well  and  will  "  come  true  to  seed."  More  important 
results  may  be  expected  in  the  future  from  hybridizing  wheats 
than  any  yet  attained.1 

1  The  literature  of  plant  breeding  is  extensive  and  rapidly  increasing.  An 
excellent  general  account  of  the  subject  and  full  bibliography  is  contained 
in  Plant  Breeding  by  L.  H.  Bailey,  The  Macmillan  Company,  New  York  and 
London,  1906. 

A  valuable  summary  of  the  main  topics  of  plant  breeding  is  contained  in 
Bulletin  No.  29,  1901,  of  the  Division  of  Vegetable  Physiology  and  Pathology 
of  the  United  States  Department  of  Agriculture. 

Much  information  is  also  given  in  Hugo  de  Vries,  Species  and  Varieties : 
their  Origin  by  Mutation,  Open  Court  Publishing  Company,  Chicago,  1905. 

Other  publications  of  the  United  States  Department  of  Agriculture  on 
plant  breeding  are  : 

For  corn,  Farmer's  Bulletin  No.  229,  1905. 

For  wheat,  Bureau  of  Plant  Industry,  Bulletin  No.  78,  1905. 

Division  of  Vegetable  Physiology  and  Pathology,  Bulletin  No.  24,  1900. 
The  publications  of  most  of  the  Agricultural  Experiment  Stations  contain 
much  important  material  for  the  discussion  of  plant  breeding.    A  few  of  these 
are  as  follows : 

For  corn,  University  of  Illinois  Agricultural  Experiment  Station,  Circular 

No.  74,  1904 ;  Bulletins  Nos.  55,  82,  1902 ;  87,  1903 ;  100,  1905. 
Ohio  Agricultural  Experiment  Station,  Bulletin  No.  140,  1903. 
Kansas  Agricultural  College,  Bulletin  No.  107,  1902. 
Nebraska  Agricultural  Experiment  Station,  Bulletin  No.  91,  1905. 
For  wheat,  University  of  Minnesota  Agricultural  Experiment  Station, 

Bulletin  No.  62,  1899. 

Ohio  Agricultural  Experiment  Station,  Bulletin  No.  165,  1905. 
The  authors  wish  to  express  their  obligations  to  all  the  authorities  above- 
mentioned.    They  have  also  to  thank  Assistant  Secretary  Willet  M.  Hays, 
of  the  Department  of  Agriculture,  for  his  kindness  in  reading  and  copiously 
annotating  the  present  chapter. 


CHAPTEK   XLI 
SOME   USEFUL   PLANTS   AND   PLANT   PRODUCTS 

490.  Economic  botany.    The  branch  of  the  science  which 
treats  of  the  uses  of  plants  to  man  is  called  economic  botany. 
Since  whole  industries  like  agriculture,  lumbering,  paper  making, 
and  a  multitude  of  others  are  concerned  with  the  utilization  of 
plants  or  parts  of  plants,  the  subject  is  a  most  extensive  one  and 
can  only  be  outlined  in  a  general  text-book  of  botany. 

A  partial  classification  of  useful  plant  products  may  be  sug- 
gested, dividing  them  into 

1.  Food  products  for  human  use. 

2.  Medicinal  plants  and  plant  products. 

3.  Food  products  for  domestic  animals. 

4.  Plants  used  as  fertilizers. 

5.  Plant  products  used  in  chemical  and  other  manufactures,  as  tan- 
ning, dyeing,  etc. 

6.  Plant  fibers  and  related  products. 

7.  Timber. 

8.  Fuel. 

9.  Ornamental  plants. 

In  general  only  those  members  of  the  classes  above  given 
which  are  of  considerable  importance  in  our  own  country  will 
be  mentioned  in  this  chapter. 

1.  FOOD  PRODUCTS  FOR  HUMAN   USE 

491.  The  grains  form  the  most  important  part  of  our  vege- 
table food ;  they  are  the  fruits  of  the  cereals,  or  food-producing 
grasses,  and  for  this  and  other  reasons  the  grasses,  which  in 
all  number  about  3500  species,  are  more  useful  to  man  than 
any  other  family  of  plants.    The  principal  genera  of  cereals  are 

514 


THE   GRAINS 


515 


wheat,  oats,  rye,  barley,  rice,  and  Indian  corn.  Most  of  the 
cereals  are  grass-like  herbs  of  moderate  height,  but  corn  varies 
much  in  size,  from  some  of  the  dwarf  varieties  of  pop  corn 
not  more  than  two  feet  high  to  the  twenty-foot  field  corn  of 
the  rich  river  bottoms  of  the  Middle  West.  All  the  grains  have 
many  varieties,  but  these  are  most  familiar  in  the  various  sorts 
of  wheat  —  hard,  soft,  red,  white,  bearded,  beardless,  and  so  on 
—  and  in  the  many  qualities  of  grain  of  Indian  corn  (see 
Chapter  XL). 

Wheat  is  the  most  important  of  the  cereals,  on  account  of 
its  palatableness,  high  food  value,  and  ready  digestibility.    None 


FIG.  383.    A  cornfield  in  Missouri 
After  Frye 

of  the  other  grains  yield  a  flour  which  is  as  well  adapted  for 
bread  making  as  wheat  flour.  Eice  is  readily  digestible,  but  is 
inferior  to  most  grains  in  the  relative  proportion  of  proteids  to 
other  ingredients  ;  and  oats,  rye,  barley,  and  Indian  corn,  as  usu- 
ally prepared,  are  somewhat  difficult  of  digestion.  Corn  meal 
when  imperfectly  cooked,  and  eaten  to  the  exclusion  of  other 
food,  has  often  given  rise  in  northern  Italy  to  a  much-dreaded 
disease  known  as  pellagra. 

The  United  States  is  the  leading  wheat-  and  corn-raising 
country,  producing  more  than  one  fourth  of  the  total  world's 
crop  of  the  former  grain  and  four  fifths  of  the  latter. 


516  USEFUL  PLANTS  AND  PLANT   PRODUCTS 

Any  of  the  grains  may  be  made  to  yield  starch  for  food  or 
for  laundry  or  manufacturing  purposes,  but  the  greater  part  of 
that  produced  in  this  country  is  obtained  from  corn,  which 
contains  about  60  per  cent  of  it.  Moldy  or  otherwise  damaged 
grain  can  be  utilized  to  some  extent  in  starch  making. 

Corn  also  contains  in  the  embryo  of  the  grain  from  3  to  6 
or  more  per  cent  of  oil,  which  is  now  largely  extracted  for  use 
as  food  and  for  various  manufacturing  purposes. 

492.  Leguminous  seeds.    Several  kinds  of  seeds  of  the  pea 
family  (Leguminosce) — an  immense  family,  comprising  some  7000 
species  —  are  important  articles  of  food.    Beans,  as  every  one 
knows,  are  used  as  food  in  all  stages,  from  the  time  when  the 
pods  are  half  grown  until  the  seeds  are  entirely  ripe  and  dry. 
In  the  latter  condition,  when  properly  cooked,  they  constitute 
one  of  the  cheapest  and  most  concentrated  forms  of  proteid  food. 

Peas,  whether  "  green  "  or  dry,  have  much  the  same  nutritive 
value  as  beans  in  the  same  stage  of  maturity.  Various  canned 
products  of  beans  and  of  peas  are  now  prepared  on  a  large  scale. 

Peanuts  are  the  seeds  of  a  leguminous  plant  which  forces  its 
growing  pods  underground,  where  they  remain  during  and  after 
the  process  of  ripening.  The  domestic  consumption  of  these  seeds 
is  large,  and  they  constitute  a  considerable  article  of  export. 

Several  other  kinds  of  leguminous  seeds,  such  as  broad  bean?, 
lentils,  and  chick  peas,  are  extensively  used  as  foods  in  parts  of 
Europe,  but  are  not  as  yet  important  articles  of  diet  in  our  own 
country. 

493.  Other  seeds.    The  remaining  kinds  of  seeds  which  are 
important  as  food  are  mostly  known  as  nuts.    Some  of  these 
are  really  drupes,  like  the  cocoanut  and  the  walnut  (Sec.  183, 
Fig.  166,  V),  while  others,  like  the  Brazil  nut  and  chestnuts,  are 
seeds. 

From  the  palm  family,  which  is  of  supreme  importance 
within  the  tropics,  only  one  kind  of  so-called  nut,  the  cocoa- 
nut,  is  commonly  in  use  among  us.  The  abundant  endosperm 
of  its  seed  is  largely  eaten  raw  and  much  used  in  cookery. 


NUTS  AND  OTHER  SEEDS 


517 


Three  rather  closely  related  families  of  trees  —  the  walnuts, 
the  birches,  and  the  beeches  —  furnish  most  of  our  edible  nuts. 
From  the  first  come  walnuts,  butternuts,  pecans,  and  hickory 
nuts ;  from  the  second,  hazelnuts  and  filberts ;  and  from  the 
third,  beechnuts  and  chestnuts. 

The  rose  family  furnishes  almonds,  which  are  technically 
drupes,  closely  related  to  peaches  (Fig.  164). 

Brazil  nuts  are  the  seeds  of  lofty  South  American  trees  of  a 
tropical  family  allied  to  the  mangroves  and  the  myrtles. 


FIG.  384.    A  grove  of  cocoa  palms  in  the  Philippines 
After  Frye 

494.  Chocolate,  tea,  and  coffee.  These  familiar  substances 
are  derived  from  plants  of  three  different  families,  the  first 
two  being  somewhat  nearly  related  tropical  or  sub-tropical 
ones. 

Chocolate  consists  of  the  ground  or  crushed  seeds  of  the 
cacao  tree,  a  native  of  Mexico,  now  widely  cultivated  through- 
out the  tropics.  Eemoval  of  a  large  part  of  the  aromatic  fat 
known  as  cacao  butter,  which  is  considerably  used  in  medicine, 
leaves  cocoa,  which  forms  for  some  people  a  more  digestible 
beverage  than  chocolate. 


518 


USEFUL  PLANTS  AND  PLANT  PRODUCTS 


Tea  is  made  from  the  leaves  of  a  shrub  long  cultivated  in 
China  and  Japan,  and  now  also  in  India,  Ceylon,  and  elsewhere. 
Unlike  chocolate,  tea  has  no  food  value,  but  is  a  mild  stimulant. 
Coffee  is  made  from  the  seeds  of  a  small  tree  widely  culti- 
vated in  hot  countries  and  belonging  to  the  madder  family. 

The  seeds  are  pro- 
duced in  red  ber- 
ries, which  are 
thickly  clustered 
about  the  twigs  of 
the  tree.  Coffee 
has  only  a  trifling 
food  value,  but  is 
a  vigorous  stimu- 
lant, reenforcing 

the  action  of  the 

heart. 

495.  Fruits 
with  fleshy  pulp. 

The  kinds  of  fruit 
with  fleshy  pulp, 
some  eaten  raw 
and  others  requir- 
ing cooking,  are 
so  numerous  that 
they  can  only  be 
FIG.  385.  A  flowering  twig  of  the  coffee  tree  mentioned  under 

Two  thirds  natural  size,  with  fruit  /  and  /*  and  seeds  s,     the    families    to 
natural  size.  -  After  Wossidlo  which  theybelong. 

From  the  palms  are  obtained  dates,  which  are  technically 
berries  with  a  very  hard  seed.  In  the  arid  portions  of  Africa 
and  northwestern  Asia,  where  they  grow,  they  are  of  the  first 
importance  as  food.  Successful  attempts  are  now  in  progress 
to  introduce  the  culture  of  the  date  palm  into  the  desert  regions 
of  the  extreme  southwestern  United  States. 


PLATE  XIII.   A  tropical  forest  in  the  Philippines,  mainly  palms 
After  F.  W.  Atkinson 


FRUITS  WITH   FLESHY  PULP 


519 


From  the  pineapple  family  our  only  edible  fruit  is  the  pine- 
apple, largely  cultivated  in  Florida  and  the  West  Indies. 

The  banana  family  is  a  very  small  one,  but  exceedingly 
important,  since  it  furnishes,  in  the  shape  of  bananas,  the  prin- 
cipal subsistence  of  multitudes  of  the  inhabitants  of  the  tropics. 
The  plant  is  herbaceous,  but  sometimes  grows  to  the  height  of 
forty  feet,  with  enormous  leaves.  It  is  extraordinarily  produc- 
tive, so  that  a  few  square  rods  of  good  soil  set  with  banana 
plants  will  supply  the  fruit  for  an  entire  family. 
Our  importation  of  bananas  is  very  large  and 
rapidly  increasing,  and  what  was  once  an  arti- 
cle of  luxury  or  a  curiosity  is  now  the  staple 
fruit  for  the  entire  year  in  most  of  our  mar- 
kets. The  principal  supply  comes  from  the 
West  Indies  and  Central  America,  but  bananas 
are  somewhat  cultivated  in  the  extreme  south- 
ern portions  of  the  United  States. 

The  mulberry  family  supplies  the  breadfruit, 

which  constitutes  the  most  important  food  of 

i          £  ^u     •  i    i  v  £  ^  ^ 

great  numbers  of  the  inhabitants  of  the  south 

Pacific  Islands.    Our  only  fruits  of  this  family 

are  the  mulberry  and  the  fig.    Most  of  our  figs   one  fourth  nat- 

are  still  imported,  but  their  culture  has  recently       ural  ^ize-  ~~  After 

become  a  considerable  industry  in  California, 

since  the  variety  which  can  be  dried  for  shipment  is  now  suc- 

cessfully cultivated  there. 

Two  closely  related  groups,  the  saxifrage  family  and  the 
rose  family,  furnish  a  large  proportion  of  all  our  true  berries, 
and  some  edible  fruits  which  are  not  berries.  From  the  former 
family  are  obtained  currants  and  gooseberries.  The  rose  family 
consists  of  five  sub-families.  Of  these  the  apple  subdivision 
furnishes  quinces,  pears,  and  apples  ;  the  rose  subdivision  fur- 
nishes strawberries,  blackberries,  and  raspberries  ;  and  the  plum 
subdivision  furnishes  plums,  cherries,  peaches,  apricots,  and 
nectarines. 


A  cacao 
d  cut       n  to 

show  the  seeds 


520          USEFUL  PLANTS  AND  PLANT  PRODUCTS 

The  rue  family  contains  a  rather  small  number  of  trees  and 
shrubs,  with  only  two  common  genera,  the  prickly  ash  and  the 
hop  tree  in  temperate  North  America,  and  comprises,  among 
others,  the  orange  sub-family.  Under  this  is  found  the  genus 
Citrus,  which  embraces  all  the  citrous  fruits.  The  species  and 
varieties  which  are  found  in  our  markets  may  be  classed  as 
oranges,  grape  fruit,  and  lemons. 

Most  of  our  oranges  are  now  of  American  growth,  coining 
from  California  or  Florida,  and  many  of  the  very  large  fruited 
species  of  Citrus  from  Polynesia,  variously  known  as  pomelo 
and  grape  fruit,  are  raised  in  both  these  states,  while  some  are 
also  imported  from  the  West  Indies.  The  best  lemons  are  im- 
ported from  the  Mediterranean  coast,  largely  from  Sicily. 

The  grape  family  numbers  about  300  species  of  climbing 
shrubs.  Only  the  grape  genus  Vitis  is  a  source  of  edible  fruits, 
—  the  berries  so  familiar  as  fresh  grapes  or  raisins. 

Of  these  there  are  two  principal  types,  one  comprising  the 
European  (Malaga  and  other)  varieties  with  solid  pulp,  found 
also  in  such  California  varieties  as  the  Tokay  grape,  all  of  which 
are  descended  from  one  European  species.  The  other  type  is 
the  one  with  soft  pulp,  readily  separated  from  the  skin,  such 
as  the  Catawba,  Delaware,  Isabella,  and  Concord  varieties.  These 
have  to  some  extent  been  introduced  into  Europe,  but  are 
descendants  of  native  American  species.  Grapes  are  consider- 
ably cultivated  in  most  of  the  states,  but  nowhere  else  so  exten- 
sively as  in  California,  where  they  are  raised  for  wine  making,  for 
the  manufacture  of  raisins,  and  for  shipment  in  a  fresh  condition. 

The  heath  family  supplies  berries  of  several  species,  such  as 
the  familiar  cranberries  and  the  blueberries  and  huckleberries, 
which  are  largely  gathered  for  the  market  in  several  of  the 
northeastern  states,  particularly  in  Maine,  and  are  somewhat 
extensively  canned. 

From  the  olive  family  (mostly  sub-tropical  trees  and  shrubs) 
are  obtained  olives,  which  constitute  a  table  delicacy,  while 
the  oil  is  a  highly  valuable  food. 


EDIBLE  LEAVES  AND   SHOOTS  521 

From  the  nightshade  family,  many  of  which  are  poisonous 
plants,  we  get  several  large  edible  fruits  (true  berries,  though 
they  are  not  popularly  so  called),  —  the  ground  cherry,  or  straw- 
berry tomato  (Physalis),  the  pepper  (Capsicum),1  the  egg  plant, 
and  the  tomato. 

The  gourd  family  furnishes  all  the  melons,  cucumbers, 
squashes,  and  pumpkins. 

496.  Edible  leaves  and  shoots.    Only  a  few  of  the  articles  of 
diet  under  this  head  have  much  commercial  importance  or  form 
a  notable  part  of  the  subsistence  of  people  in  any  portion  of  the 
country. 

From  the  lily  family  we  get  asparagus ;  from  the  pigweed 
family,  spinach  ;  from  the  mustard  family,  water  cress,  cabbage, 
cauliflower,  and  Brussels  sprouts ;  from  the  parsley  family,  celery ; 
and  from  the  Composite,  lettuce  and  globe  artichokes  (Cynara). 

497.  Edible  bulbs,  rootstocks,  tubers,  and  roots.    As  is  else- 
where explained  (Sec.  66),  reserve  material  is  often  stored  in 
underground  portions  of  the  plant  body.    The  number  of  vege- 
tables derived  from  these  is  not  very  large,  but  they  constitute 
a  considerable  part  of  the  food  of  people,  especially  in  temperate 
and  cold  climates. 

From  the  lily  family  onions  are  obtained,  from  the  yam 
family  yams,  from  the  pigweed  family  beets,  from  the  mustard 
family  turnips  and  radishes,  from  the  parsley  family  carrots 
and  parsnips,  from  the  morning-glory  family  sweet  potatoes, 
from  the  nightshade  family  potatoes,  and  from  the  Composite 
salsify  and  Jerusalem  artichokes  (Helianihus). 

498.  Starch  and  sugar  from  stems  and  roots.    Sago  is  the 
purified  starchy  pith  of  small  palms,  natives  of  Siam  and  of 
some  of  the  Malayan  Islands.    A  portion  of  the  supply  also 
comes  from  West  Indian  cycads  (Sec.  346). 

Tapioca  is  a  starchy  substance  obtained  from  the  grated  roots 
of  plants  of  the  spurge  family  (Euplwrbiacece),  cultivated  in 
tropical  America  and  the  West  Indies. 

1  This  is  not  a  pulpy  fruit. 


522 


USEFUL  PLANTS  AND  PLANT  PRODUCTS 


Arrowroot  is  a  very  pure  starchy  food   obtained  from   the 
rootstocks   of   plants   of   two   or   three   tropical   families,  espe- 
cially the  arrowroot  family 
(Marantacecc). 

Sugar  is  largely  manu- 
factured in  Europe  and  to 
some  extent  in  the  United 
States  from  the  juice  of  the 
sugar  beet.  The  remainder 
of  the  world's  supply  of 
sugar  comes  from  the  stem 
of  the  sugar  cane,  a  grass 
which  grows  to  a  height 
of  ten  feet  or  more.  It  is 
somewhat  cultivated  in 
Louisiana,  but  much  more 
extensively  in  the  West 
Indies,  Java,  and  the 
Hawaiian  Islands. 


2.  MEDICINAL  PLANTS 
AND  PLANT  PROD- 
UCTS 

499.  The  study  of  me- 
dicinal plants  is  a  special 
subject,  forming  an  im- 
portant part  of  the  course 
in  every  college  of  phar- 
macy. Only  a  few  words  can  be  given  to  the  topic  in  this 
chapter. 

Very  many  of  the  families  of  angiosperms  contain  species 

used  in  medicine.1    In  some  cases,  as  in  the  lily  family,  the 

pea  family  (which  furnishes  sixteen  remedies),  the  mint  family, 

1  In  the  United  States  Pharmacopoeia  sixty-seven  families  are  represented. 


FIG.  387.    Sugar  cane  (Saccharum) 
Much  reduced.  —  After  Wossidlo 


FOOD  PRODUCTS  FOR  DOMESTIC   ANIMALS        523 

and  the  nightshade  family,  medical  properties  are  quite  gener- 
ally distributed  throughout  the  whole  family  or  through  certain 
sections  of  it.  In  other  cases,  as  in  the  poppy  family  (which 
yields  opium  and  morphia),  the  family  Erytkroxylacece  (which 
yields  cocaine),  and  the  figwort  family  (which  yields  digitalis) 
only  one  important  remedy  or  group  of  remedies  occurs.  The 
properties  of  many  medicinal  plants  were  discovered  by  acci- 
dent in  primitive  times,  while  others  have  had  their  value 
established  only  as  a  result  of  careful  experiments  on  man  and 
the  lower  animals. 

3.  FOOD  PRODUCTS  FOR  DOMESTIC  ANIMALS 

500.  The  most  important  herbivorous  domestic  animals  — 
cattle,  horses,  and  sheep  —  consume'large  quantities  of  the  less 
expensive  grains,  and  in  general  the  roots  and  tubers  which  are 
useful  for  human  food  are  readily  eaten  by  these  animals. 

A  large  proportion  of  the  grasses  are  utilized  by  grazing  ani- 
mals or  fed  as  hay.  Many  plants  of  the  pea  family,  particu- 
larly alfalfa,  the  clovers,  soy  beans,  and  cow  peas,  are  eaten  by 
domestic  animals. 

Both  grasses  and  other  plants  are  cut  and  fed  to  cattle  and 
horses,  while  fresh,  as  for  age.  Large  quantities  of  "  corn  fodder  " 
are  used  in  this  way  in  many  parts  of  the  country,  and  the 
stems  and  leaves  of  corn  are  also  cut  up,  placed  in  large  tanks 
called  silos,  allowed  to  ferment,  and  then  fed  to  cattle  through- 
out the  winter. 

Certain  by-proc\ucts  of  manufacturing  processes  are  of  much 
value  for  cattle  food.  Among  the  most  important  of  these  are 
linseed  meal  and  cotton-seed  meal,  which  are. rich  in  proteids  and 
still  retain  some  oil  after  the  greater  part  of  it  has  been  ex- 
tracted by  the  most  powerful  pressure  available.  The  refuse 
grains  from  breweries  and  the  sloppy  boiled  corn  meal  from 
distilleries  are  in  a  wet  state  extensively  fed  to  cattle  and 
hogs,  but  are  injurious  if  used  alone.  They  are  also  dried  for 


524  USEFUL   PLANTS  AND  PLANT  PRODUCTS 

shipment.  The  refuse  from  beet  sugar  manufacturing  establish- 
ments is  used  in  a  wet  condition  for  cattle  feeding,  and  is  also 
dried  and  shipped. 

Some  seeds  not  eaten  by  man  are  highly  valuable  when  fed 
to  the  lower  animals.  Acorns  and  beechnuts,  for  example,  in 
some  of  the  wooded  portions  of  the  southern  Middle  States,  fur- 
nish a  considerable  part  of  the  subsistence  of  droves  of  hogs. 

4.  PLANTS  USED  AS  FERTILIZERS 

501.  For  centuries  the  advantage  of  plowing  under  growing 
crops   as  a  means  of  enriching  worn-out  land  has   been   well 
recognized.    It  is  only  very  recently  that  the  exact  significance 
of  this  process  has  been  understood.    Even  now  the  details  are 
not  so  fully  worked  out  that  we  know  just  what  crop  will  yield 
the  best  results  for  every  variety  of  soil  and  climate ;  but  in  a 
general  way  it  is  established  that  leguminous  plants  are  the 
best  for  this  purpose  on  account  of  the  power  which  their  root 
tubercles    have   of  utilizing    the   nitrogen    of   the   atmosphere 
(Sec.  256).     Various   clovers   and   alfalfa    are   the    crops    most 
commonly  employed. 

5.  PLANT  PRODUCTS  USED  IN  MANUFACTURES 

502.  Under  this  head  there  is  only  space  to  mention  a  very 
few  of  the  vegetable  substances  used  in  manufacturing  processes, 
most  of  them  on  account  of  their  chemical  properties. 

Dyeing  by  means  of  vegetable  coloring  matters  is  far  less 
important  than  it  was  before  the  introduction  of  the  artificially 
prepared  aniline  colors.  These  are  so  powerful  that  it  is  more 
economical  to  use  them,  but  they  do  not  give  soft  shades.  Val- 
uable dyes,  however,  are  still  obtained  from  a  considerable  num- 
ber of  plants.  Many  of  these  belong  to  members  of  the  pea 
family,  which  furnishes  Brazil  wood  (red),  logwood  (red,  purple, 
and  black),  camwood  (red),  indigo  (dark  blue). 


PLANT   PRODUCTS   USED  IN  MANUFACTURES      525 


From  the  buckthorn  family  are  obtained  yellow  and  green 
dyestuffs,  known,  respectively,  as  Persian  berries  and  Chinese 
green  indigo. 

Varnishes  of  great  value  are  yielded  by  trees  of  the  pea  family 
(copal  varnish)  and  of  the  sumach  family  (Japanese  lacquer). 

Tanning  is  largely 
carried  on  by  aid  of 
the  bark  of  several 
species  of  oak,  of 
which  the  black  oak 
and  the  Spanish  oak 
are  two  of  the  most 
used  American  spe- 
cies. Hemlock  bark 
and  the  leaves  and 
young  twigs  of  Sicil- 
ian and  American 
species  of  sumach 
are  also  used  for 
tanning.  Other  sub- 
stances employed  for 
the  same  purpose  are 
catechu,  derived  from 
a  species  of  acacia, 
and  gambier,  derived 
from  the  evaporated 
sap  of  a  tree  of  the  ^IG-  ^^*  ^  twig  °f  ^ne  South  American  rubber 

tree  (Hevea) 

madder  family,  a  na- 
tive of  the  East  Indies.  After  Schmidt 

India  rubber  is  manfactured  from  the  sap  of  several  tropical 
trees  and  lianas.  The  principal,  one  of  these  is  the  Para  rubber 
tree  (Hevea)  of  the  spurge  family. 

Gutta-percha  is  produced  by  trees  of  the  star  apple  family 
(Sapotacece)  of  the  Malay  Archipelago,  a  family  of  much  eco- 
nomic importance. 


526  USEFUL  PLANTS  AND  PLANT  PRODUCTS 

6.  PLANT  FIBERS  AND  RELATED  PRODUCTS 

503.  Fibrous  materials  for  use  in  spinning  into  thread,  cord- 
age, and  rope,  also  for  braiding  and  weaving,  are  obtained  from 
many  parts  of  the  plant  body.  Some  of  the  most  useful  of 
these,  such  as  flax  and  hemp,  are  derived  from  the  hard  bast, 
others,  as  cotton,  consist  of  plant  hairs,  and  others  still  rep- 
resent various  structural  elements  of  the  plant. 

A  large  proportion  of  the  fibrous  materials  in  general  use 
comes  from  monocotyledonous  plants  of  several  families. 


FIG.  389.    A  Georgia  cotton  field 
After  Frye 

Several  sedges  of  the  genus  Cyperus  furnish  materials  for 
weaving,  and  East  Indian  and  Chinese  mattings  are  made  from 
species  cultivated  for  the  purpose. 

The  straw  of  various  grains  is  employed  for  braiding  into 
baskets,  mats,  hats,  and  other  articles.  A  coarse  grass  known 
as  esparto  is  largely  exported  from  Spain  and  the  North  African 
coast  for  use  in  paper  making  and  for  other  purposes. 

Many  palms  produce  valuable  fiber ;  that  of  the  husk  of  the 
cocoanut  is  largely  used  for  cordage,  mats,  brushes,  and  similar 
articles. 


PLANT   FIBERS  AND  RELATED  PRODUCTS          527 

From  material  obtained  from  the  very  young  leaves  of 
a  somewhat  palm-like  plant  (Carludovica)  the  well-known 
Panama  hats  are  woven. 

Several  plants  of  the  lily  family,  especially  the  so-called  New 
Zealand  flax  and  the  century  plant  (Fig.  391),  furnish  fibers. 


FIG.  300 

A  portion  of  a  cotton  plant  in  bloom  with  a  ripe  capsule  or  boll  b  and  seed  .s.     All 
slightly  reduced.  —  After  Wossidlo 

From  a  member  of  the  banana  family,  a  native  of  the 
Philippines,  but  cultivated  also  in  India,  is  obtained  the  ex- 
tremely valuable  manila  fiber,  one  grade  of  which  is  so  fine  as 
to  be  woven  into  delicate  shawls  and  similar  fabrics,  while 
the  coarser  kinds  are  used  in  the  manufacture  of  manila 
rope. 

Among  dicotyledonous  plants  there  are  a  considerable  number 
which  serve  as  sources  of  commercial  fibers. 


528 


USEFUL   PLANTS  AND  PLANT   PRODUCTS 


To  the  mulberry  family  belong  the  paper  mulberries,  which 
furnish  bark  from  which  the  beautiful  Japanese  paper  is  made, 
and  the  hemp  plant,  which  is  one  of  the  chief  rope-  and  cordage- 
making  materials. 

To  the  nettle  family  belongs  ramie,  an  eastern  Asiatic 
plant  cultivated  in  Jamaica  and  the  southern  United  States, 
from  which  Chinese  grass  cloth  and  other  fabrics  are  made. 

Three  closely  related  groups  of  plants  —  the  linden  family, 
the  mallow  family,  and  the  silk  cotton  family  —  yield  many 
fibrous  or  hair-like  products  of  use  for  spinning  and  weaving, 

or  for  mattress  mak- 
ing and  similar  pur- 
poses. From  the 
bark  of  trees  of  the 
first-named  family  is 
obtained  the  Russian 
bass,  or  bast,  used 
for  making  rough 
mats,  and  the  tropi- 
cal product  jute, used 
to  weave  with  silk, 
and  also  for  carpets, 
mats,  and  coarse 
bags.  From  the  hairs 
which  clothe  the  seed  of  the  cotton  plant  (the  most  important 
member  of  the  mallow  family)  all  cotton  goods  are  manufac- 
tured. Cotton  is  largely  cultivated  in  India,  Egypt,  and  our  own 
country.  It  is  an  important  crop  in  all  of  our  Gulf  states,  and 
in  Georgia  and  South  Carolina.  The  seed  hairs  of  the  tropical 
silk  cotton  trees  (Ceiba)  are  coming  to  be  much  used  in  pillows 
and  cushions  as  a  substitute  for  feathers. 

Most  vegetable  fibers,  such  as  have  been  described  in  this 
chapter,  are  useful  for  paper  making,  even  after  the  rope  or 
woven  fabrics  made  from  them  have  been  worn  until  they  are 
dropping  to  pieces.  Large  areas  of  forest,  particularly  of  spruce 


FIG.  391.    Century  plants  (Agave) 
After  Frye 


CONIFEROUS  WOODS 


529 


and  poplar  growth,  are  now  annually  cut  down  to  furnish  paper 
pulp.  It  has  been  recently  proposed  to  utilize  cotton  stems  for 
paper  pulp.  Ten  million  or  more  tons  of  the  raw  material,  worth 
nearly  a  dollar  a  ton  for  this  purpose,  are  now  annually  avail- 
able in  the  cotton-growing  states. 

7.  TIMBER 

504.  Coniferous  woods.    The  wood  of  our  cone-bearing  trees 
(mainly  of  the  pine  family)  is  generally  known  as  soft  wood, 


y         ¥.&>!>,  &W 


FIG.  392.    Forest  of  hard  or  yellow  pine  (Pinus  palustris]  on  southern 
coastal  plain  of  the  United  States 

After  Frye 

and  that  of  our  broad-leaved,  mostly  deciduous  trees  is  known 
as  hard  wood.  These  terms  are  not  quite  correct,  for  the  conif- 
erous larches  and  yews  furnish  a  harder  wood  than  that  of  such 
broad-leaved  trees  as  willows,  poplars,  tulip  trees,  and  buckeyes. 
Out  of  the  entire  timber  supply  of  the  country  more  than 
three  quarters  is  at  present  furnished  by  the  thirty-eight  or  more 


530  USEFUL  PLANTS  AND  PLANT  PRODUCTS 

species  of  cone-bearing  trees,  especially  the  pines,  which  grow 
within  our  limits. 

The  wood  of  the  white  pine  (Pinus  strobus),  remarkable  for 
its  workableness  and  freedom  from  warping  or  cracking  when 
exposed  to  the  weather,  was  for  years  the  most  important  of  all 
our  soft  woods.  Latterly,  as  the  supply  is  becoming  greatly 
lessened,  other  kinds  of  pine,  especially  the  long-leaf  pine, 
the  loblolly  pine  of  the  southeastern  states,  and  the  bull  pine 
(P.  ponderosa)  of  the  Pacific  and  Rocky  Mountain  regions,  are 
to  a  considerable  extent  taking  its  place. 

Among  the  other  most  widely  used  coniferous  woods  are  two 
species  of  true  spruce  (Picea),  the  "Douglas  spruce"  (Pseudotsuga), 
two  western  species  of  white  fir  (Abies),  the  smaller  California 
redwood  (Sequoia},  the  American  or  bald  cypress  (Taxqdium), 
and  several  distinct  kinds  of  white  cedar  (Thuya,  Chamcer 
cyparis,  and  Libocedrus).  The  cypress,  larch,  and  most  of  the 
cedars  furnish  timber  of  great  durability  when  exposed  to  the 
weather  or  buried  in  the  earth,  and  therefore  are  highly  valued 
for  posts,  telegraph  poles,  railroad  ties,  and  similar  uses. 

505.  Broad-leaved  woods.  Our  native  broad-leaved  trees  which 
furnish  wood  for  manufacturing  or  constructive  purposes  com- 
prise about  eighty  species,  a  larger  number  than  is  found  in 
any  other  equal  area  of  the  temperate  zones. 

The  principal  hard-wood  forests  are  of  oak,  though  other  valu- 
able timber  trees,  such  as  maples,  hickories,  beeches,  and  elms, 
are  usually  scattered  among  them.  Our  oak  lumber  is  of  three 
kinds,  —  white,  red,  and  live  oak.  White  oak  is  much  superior 
to  red  for  constructive  purposes  where  strength  is  important,  but 
does  not  show  so  conspicuous  a  grain  when  polished  for  cabinet 
work.  More  than  half  of  our  supply  of  hard  woods  comes  from 
various  species  of  oak. 

Next  in  importance  is  the  wood  of  the  tulip  tree  (Lirioden- 
dron),  generally  known  as  yellow  poplar,  or  whitewood.  This 
has  largely  taken  the  place  of  white  pine  in  inside  woodwork 
for  dwelling  houses  and  other  buildings. 


BROAD-LEAVED   WOODS  531 

Among  the  most  generally  useful  of  the  other  broad-leaved 
woods  may  be  mentioned  maple,  elm,  ash,  and  chestnut. 

Several  kinds  are  particularly  valued  for  their  durability  in  the 
ground;  among  these  are  chestnut,  black  locust,  and  catalpa. 

For  cabinet  work  the  most  prized  of  our  native  woods  are 
black  walnut,  cherry,  birch,  and  some  species  of  oak.  None  of 


FIG.  393.    Hickory  (hard  wood)  forest  near  southern  end  of 
Appalachian  highlands 

After  Frye 

these  is  as  beautiful  as  some  of  the  finer  imported  kinds,  such 
as  mahogany,  rosewood,  and  satinwood. 

506.  Forestry.  During  the  -time  when  the  country  was  in 
process  of  being  settled  most  portions  of  the  Atlantic  coast 
region,  and  inland  as  far  as  the  prairies  of  what  are  now  the 
states  of  Illinois  and  Minnesota,  were  covered  with  primitive 
forest.  The  most  difficult  task  of  the  settler  was  to  clear 
enough  land  for  tillage.  The  finest  timber  trees  were  destroyed 
by  hundreds  of  thousands  by  the  process  of  girdling,  that 


532 


USEFUL  PLANTS  AND  PLANT  PRODUCTS 


is,  by  cutting  away  a  ring  of  sapwood  and  allowing  the  trees 
to  die  of  starvation  and  lack  of  water.  Cultivation  was  car- 
ried on  among  them,  until  after  some  years  the  trees  would 


FIG.  394.    A  corn  field  in  a  "  deadening1'  or  girdled  forest  of 
deciduous  trees 

Modified  after  Ay  res 

fall  and  then  were  burned  to  rid  the  land  entirely  of  their 
presence.  More  than  a  century  and  a  half  of  wholesale  destruc- 
tion of  trees  has  finally  resulted  in  stripping  large  areas  of  the 
original  forest  and  preventing  the  reforestation  of  the  land,  until 


FORESTRY  533 

a  point  has  been  reached  when  it  is  difficult  to  get  lumber 
of  good  quality  for  many  of  the  most  important  purposes  for 
which  it  is  used.  In  some  parts  of  the  country  timber  is  a 
profitable  crop  to  raise,  even  if  it  has  to  be  planted  and  cared 
for  while  growing.  The  science  and  art  of  growing  timber  and 
caring  for  tracts  of  wooded  land  is  called  forestry.  Much  atten- 
tion has  long  been  paid  to  it  in  the  most  enlightened  countries 
of  Europe,  but  the  subject  is  a  comparatively  new  one  in  the 
United  States.  The  importance  of  maintaining  a  suitable  pro- 
portion of  wooded  land  in  any  region  does  not  depend  merely 
on  the  desirability  of  a  supply  of  timber.  The  water  supply  of 
lakes  and  streams,  the  retention  of  the  cultivable  layer  of  loam 
on  the  earth's  surface,  the  climate  of  any  region,  at  least  so  far 
as  the  prevention  of  severe  winds  is  concerned,  —  all  are  depend- 
ent on  the  presence  of  considerable  forest  areas. 

The  principles  of  forestry  cannot  be  laid  down  in  a  few  words, 
and  forest  management  requires  years  of  study  in  the  woods 
themselves.  The  literature  on  the  subject  is  extensive,  and 
courses  in  forestry  are  now  given  in  a  good  many  universities. 
Evidently  it  is  a  topic  of  growing  importance  in  this  country. 
A  few  useful  rules  can  be  given  here. 

1.  Tree  cutting  should  generally  be  managed  on  the   prin- 
ciple of  selecting  only  mature  trees  and  leaving  the  others  to 
grow  up  to  replace  those  cut. 

2.  Forest  fires  should  be  prevented. 

3.  Destructive  fungi  should  be  exterminated  wherever  found. 

4.  Insect  enemies  of  trees,  such  as  the  seventeen-year  locust, 
the  various  caterpillars,  and  boring  insects,  should  be  destroyed. 

5.  Sheep  and  cattle  should  never  be  pastured  in  woods  where 
they  can  do  harm  by  killing  young   seedling   trees   or  other 
useful  undergrowth. 

6.  Tree  planting  should  be  carried  on  whenever  it  can  be 
made  to  utilize  lands  not  needed  for  other  purposes,  and  the 
species  planted  should  be  chosen  with  extreme  care  to  meet  the 
requirements  of  the  soil  and  climate. 


534  USEFUL  PLANTS  AND  PLANT  PRODUCTS 

8.  FUEL 

507.  Nearly  all  fuel  is  of  vegetable  origin.    In  most  civilized 
countries  to-day  the  principal  fuel  supply  consists  of  various 
kinds  of  coal,  that  is,  of  vegetable  matter  which  has  been  buried 
in  the  earth  for  ages  and  undergone  many  changes  (Sec.  330). 

Peat,  the  consolidated  material  left  after  the  partial  decay  of 
certain  bog  mosses  (Sec.  292),  in  some  countries  forms  a  consid- 
erable part  of  the  available  fuel,  and  the  deposits  in  the  north- 
ern United  States  are  of  some  importance. 

Wood,  in  portions  of  the  country,  is  still  the  principal  fuel. 
Certain  varieties  are  preferred  for  household  use  on  account  of 
their  furnishing  good  beds  of  glowing  coals,  or  for  burning  in 
open  fires  on  account  of  their  freedom  from  any  tendency  to 
snap.  But  in  general  the  fuel  value  of  thoroughly  seasoned 
wood  is  nearly  proportional  to  its  weight  per  cubic  foot,  that  is 
to  say,  the  very  heaviest  woods,  such  as  hickory,  the  white  oaks, 
black  locust,  and  some  kinds  of  ash,  are  worth  most  for  heating. 

Other  parts  of  plants  besides  wood  are  used  to  some  extent 
for  fuel.  In  large  tanneries  the  spent  bark  is  often  compressed 
to  extract  most  of  the  water  and  then  burned.  Corncobs  are 
often  burned  in  stoves  and  under  steam  boilers.  In  treeless 
regions  twisted  ropes  of  straw  are  used  as  fuel. 

9.  ORNAMENTAL  PLANTS 

508.  Our  ornamental   plants  may  be  roughly  classed  into 
shade  trees,  shrubs,  herbaceous  perennials,  and  annuals.    The 
total  number  of  species  and  varieties  cultivated  in  the  United 
States  runs  far  into  the  thousands,  but  in  many  cases  florists' 
varieties  are  distinguished  from  one  another  only  by  color  or 
some  other  comparatively  unimportant  characteristic. 

Most  of  our  cultivated  ornamental  plants  are  of  foreign  origin, 
and  representatives  of  almost  all  parts  of  the  earth  except  the 
arctic  regions  are  found  among  them.  In  a  few  instances  native 


ORNAMENTAL   PLANTS  535 

species  are  familiar  occupants  of  our  flower  gardens,  as,  for 
example,  the  native  azaleas  and  Rhododendrons,  the  bee  balm, 
California  poppy,  evening  primrose,  Mariposa  lily,  Missouri 
currant,  purple  flowering  raspberry  (Rubus  odoratiis),  cone 
flower  (Rudbeckia),  snow  on  the  mountains  (Euphorbia),  and 
wild  cucumber. 

Some  of  the  families  which  contribute  most  largely  to  our 
lists  of  cultivated  flowers  are  the  lily,  the  amaryllis,  the  pink, 
the  crowfoot,  the  rose,  the  pea,  the  geranium,  the  heath,  the 
mint,  and  also  the  composite  family. 


APPENDIX 


[Additional  illustrations,  chiefly  for  use  with  a  flora  in  determination 
of  species'] 


I.   LEAF  FORMS 


FIG.  1.   General  outline  of  leaves 

a,  linear ;   6,  lanceolate ;    c,  wedge-shaped ;   d,  spatulate ;   e,  ovate ;  /,  obovate ; 
g,  kidney-shaped ;  h,  orbicular ;  i,  elliptical 


537 


538 


APPENDIX 


/  g  hi 

FIG.  2.   Tips  of  leaves 

a,  acuminate  or  taper-pointed;  b,  acute;  c,  obtuse;  d,  truncate;  e,  retuse;  /, 
emarginate  or  notched;  y  (end  leaflet),  obcordate;  h,  cuspidate,  —  the  point 
sharp  and  rigid;  i,  mucronate, — the  point  merely  a  prolongation  of  the 
midrib 


FIG.  3 

J,  shapes  of  bases  of  leaves:  1,  heart-shaped;  2,  arrow-shaped;  3,  halberd-shaped. 
B,  peltate  leaf  of  troprcolum 


APPENDIX 


539 


II.    FORMS  OF  SYMPETALOUS  COROLLA 


FIG.  4 

Bell-shaped  corolla  of 
bell-flower  (Cam- 
panula) 


FIG.  f, 

Salver-shaped  corolla  of 
jasmine  (magnified) 


FIG.  0 

Wheel-shaped 
corolla  of 
potato 


FIG.  7 

Tubular  corolla,  from 
head  of  bachelor's 
button 


FIG.  8 

Labiate  or  ringent 
corolla  of  dead 
nettle 


INDEX 


References  to  illustrations  are  indicated  by  stars.    App.  indicates  Appendix 


Absorption  of  carbon  dioxide,  107, 
108 

Absorption  of  water  by  roots,  28 

Absorption,  root,  30 

Absorption,  selective,  36 

Acacia,  leaf  of,  98* 

Accessory  buds,  82*,  84* 

Accessory  fruits,  146,  149*,  150* 

Actinomorphic,  125 

Acuminate,  App.  I 

Acute,  App.  I 

Adaptations  to  conditions  of  exist- 
ence, 496,  497 

Adder's-tongue,  324* 

Adventitious  buds,  83,  87* 

Adventitious  roots,  19 

^Ecidium,  201,  262* 

Aerial  roots,  19-21* 

Agaricus,  267 

Age  of  trees,  46 

Aggregate  fruits,  146,  149*,  150* 

Ailanthus  twig,   81* 

Air  chamber,  103*,  104* 

Air  passages,  in  Hippuris  stem,  111* 

Air  plants,  47* 

Air,  relation  to  germination,  7 

Akene,  147* 

Albugo,  245* 

Albuminous  substances,  9 

Alga-like  fungi,  239-247* 

Alga-like  fungi,  summary  of,  247 

Algae,  172,  173-226* 

Algae,  distribution  of,  on  rocks,  215* 

Algae,  evolution  of,  225,  226 

Algae,  life  histories  of,  221,  222 

Alpine  vegetation,  484-487*,  493 

Alternate  branching,  41* 

Alternate  leaves,  81* 

Alternation  of  generations,  220,  278, 
345-350 

Alternation  of  generations,  proto- 
plastic basis  of,  345-348 

Amanita,  267 


Amoeba,  158*,  159 
Anabaena,  176,  177 
Angiospermae,  376-388* 
Angiosperm  flower,  376-379* 
Angiosperm,  life  history  of,  388 
Angiosperms,  128,  376-388* 
Angiosperms,  classification  of,  397 
Animal  food,  need  of,  412 
Animals     and     plants,    distinctions 

between,  168,  169 

Animals,  defenses  against,  413-419* 
Annual  growth,  definite,  42 
Annual  growth,  indefinite,  42 
Annual  ring,  66*,  68* 
Annuals,  46 

Anther,  127*,  139,  140*,  378-380* 
Anther,  modes  of  opening,  140* 
Antheridium,  189*,  204,  275*,  276 
Antherozoid,  189,  204 
Anthoceros,  288*,  289* 
Anthocerotales,  288*,  289* 
Antipodal  cells,  144*,  383* 
Antitoxins,  236 
Ant  plants,  414*,  415 
Ants  plant  seeds,  446* 
Apetalous,  123,  124* 
Apogamy,  244,  319,  320 
Apospory,  320 

Apples,  selection  among,  503* 
Aquatic  plants,  459-462*,  488 
Archegonium,  276* 
Archichlamydeae,  897,  398 
Arch  of  hypocotyl,  6*,  12*,  13 
Arctic  vegetation,  484 
Arctic  willow,  484* 
Aristolochia  stem,  bundle  of,  61* 
Aristolochia  stem,  cross  section  of, 

60*,  61* 

Arrangement  of  leaves,  94-101* 
Arrow-shaped,  App.  I 
Artemisia,  480,  493,  494 
Ascocarp,  248,  250 
Ascomycetes,  248-257* 


541 


542 


INDEX 


Ascus,  248,  249*,  251* 

Asexual  generation,  278,  304,  342, 

345-350 

Ash  tree,  naturally  grafted,  69* 
Asparagus,  55* 
Aspergillus,  250* 
Aspidium,  308*,  314* 
Assimilation,  107,  112,  115,  163 
Associations,  plant,  474,  475 
Autumn  leaves,   coloration  of,   121, 

122 

Auxospore,  196* 
Axillary  bud,  81*,  82* 
Axillary  inflorescence,  132* 

Bacteria,  228-237*,  238 

Bailey,  L.  H.,  448 

Barberry,  spiny  leaves  of,  416* 

Bark,  59,  60*,  65-67*,  71,  72,  366*, 

368 

Bark  cells,  25* 
Basidia  fungi,  258-271* 
Basidiomycetes,  258-271* 
Basidium,  258-267* 
Bast,  312*,  367 
Bast  bundle,  25*,  62* 
Bast,  soft  (sieve  tubes),  61-63* 
Batrachospermum,  217 
Beans,  selection  among,  504 
Bees,  422*,  423,  426* 
Beggar's  ticks,  444* 
Bell-shaped,  App.  II 
Belt's  bodies,  414*,  415 
Berry,  148*,  150* 
Biennial,  26,  46 
Bilaterally      symmetrical      flowers, 

124*,  125* 

Bilaterally  symmetrical  leaves,  88 
Birch,  branching  of,  46* 
Bird  pollination,  428 
Bisexual,  123* 
Bitter  roots,  25 
Bitter  seeds,  11    * 
Black  knot,  252* 
Bladderwort,  437* 
Blights,  244-247* 
Blister  blight,  245* 
Blue-green  algse,  174-178* 
Blue-green  algse,  life  habits  of,  177, 

178 

Blue-green  algse,  summary  of,  178 
Bog  zonation,  476-478* 
Boletus,  265*,  267 
Botany,  definition  of,  1 
Botany,  economic,  500-535* 


Botany,  economic,  definition  of,  3 

Botany,  systematic,  definition  of,  2 

Hotrychium,  324* 

Botrydium,  200* 

Box  elder,  buds  of,  82* 

Box  elder,  radial  and  cross  sections 

of  stem  of,  61* 
Brace  roots,  21* 
Bract,  133*,  138* 
Branched  leaves,  91-93* 
Branches  formed  from  adventitious 

buds,  87 

Branching,  alternate,  41*,  94* 
Branching    and    leaf    arrangement, 

41-45* 

Branching,  opposite,  41* 
Branch  thorn,  44* 
Breeding,  plant,  500-513* 
Brown  algae,  205-213* 
Brown  algse,  life  habits  of,  205,  206 
Brown  algse,  summary  of,  213 
Bryales,  293-301* 
Bryophyta,  275-305* 
Bryophytes,  275-305* 
Bryophytes,  evolution  of,  302 
Bryophytes,  origin  of,  302 
Bryophytes,  summary  of,  303,  304 
Buckeye,  bud  of,  80* 
Budding,  238* 
Buds,  80-87* 

Buds,  adventitious,  83.  87* 
Buds,  dormant,  87 
Buds,  naked,  80,  81* 
Buds,  position  of,  82*.  83,  87* 
Buds,  structure  of,  80-8(5* 
Bud-scale  scar,  40* 
Bud  scales,  80*,  83 
Bulb,  53* 

Bulb,  hyacinth,  53* 
Bulblets,  438 

Bulrush,  cross  section  of  stem  of,  58* 
Burbank,  Luther,  510 
Burs,  442-444* 

Buttercup,  leaf  of,  91*.  104-106* 
Butternut,  buds  of,  82* 
Buttons,  266* 

Cactus,  54*,  463* 

Caladium,  52* 

Calamites,  339,  340,  Plate  VIII 

Calcium,  28,  29 

Calyptra,  277,  295,  296*,  301* 

Calyx,  123*,  124*,  126* 

Cambium,  25*,  60-67*,  366*,  367 

Cambium  ring,  65,  67*,  71,  72,  367 


INDEX 


543 


Camptosorus,  310* 

Canal  cells,  276*,  277 

Canna,  381* 

Canna,  parallel  veining  in,  89* 

Cap  (pileus)  of  gill  fungi,  266* 

Capsule,  146,  150* 

Carbon,  28,  29 

Carbon  dioxide,  7,  107,  108,  113,  115 

Carbon  dioxide,  absorption  of,  107, 
108,  342 

Carboniferous  Age,  339-341 

Carnivorous  plants,  409-412* 

Carpel,  127,  358,  369-371*,  380*,  381 

Carpogonium,  217* 

Carpospore,  217* 

Carrion  fungi,  269 

Castor  bean,  germination  of,  6* 

Castor-oil  plant,  early  history  of 
stem,  65* 

Castor-oil  plant,  fibro-vascular  bun- 
dle of,  65* 

Caterpillar  and  grub  fungi,  253* 

Catharinea,  293* 

Catkin,  134* 

Cedar  apples,  264 

Cedar,  red,  446* 

Cell,  34-39*,  129,  156-167* 

Cell  contents,  34,  35*,  160*,  161* 

Cell  division,  34,  159,  164,  165* 

Cell  growth,  164 

Cell  reproduction,  164 

Cell  sap,  38*,  160 

Cell  structure  of  moss  leaf,  161* 

Cell  structure  of  pond  scum,  160* 

Cell  theory  of  organization,  165-167 

Cell  turgor,  161    . 

Cell  wall,  34,  159,  160*,  161* 

Cells,  starch  in,  9* 

Cellulose,  34,  38,  159 

Central  cylinder,  24* 

Central  placenta,  129*,  130 

Cetraria,  256*,  257 

Chsetophoraceae,  190*,  191 

Chara,  202* 

Charales,  201,  202* 

Cherry,  buds  in  axils  of  leaves,  81* 

Cherry  twig,  84* 

Chlamydomonas,  182* 

Chlorophycese,  179-204* 

Chlorophyll,  105,  109,  160 

Chlorophyll  bodies,  105,  106* 

Chloroplast,  105,  106*,  107,  160, 
161* 

Choripetalous,  126 

Chorisepalous,  126 


Chromatin,  164,  165* 

Chromatophore,  160* 

Chromatophore,  fission  of,  161*,  165 

Chromosomes,  165* 

Cilium,  170* 

Circle  (whorl),  124*,  126* 

Circulation  of  nitrogen,  231* 

Circulation  of  protoplasm,  201 

Citrous  fruits,  hybridizing  of,  511 

Cladonia,  257* 

Cladophora,  191* 

Cladophyll,  56* 

Class,  153 

Classification,  152 

Clathrocystis,  174*,  175 

Clavaria,  264* 

Claviceps,  252,  253* 

Cleistogamous  flowers,  432,  433* 

Clerodendron,  429* 

Climbing  plants,  20*,  47-50*,  92* 

Climbing  shrubs,  stem  structure,  60*, 

67 

Climbing  stems,  20*,  47-50* 
Closed  veining,  89* 
Clover  leaf,  98* 
Club  moss,  329-339* 
Cluster  cup,  261,  262* 
Clustered  roots,  23* 
Coal,  341 
Coalescence,  388 
Cocklebur,  442,  443*,  444* 
Ccelosphgerium,  175 
Ccenocyte,  197,  200,  201,  240 
Coenogamete,  242 
Coiling,  48*,  49* 
Coleochsete,  191,  192* 
Collective  fruits,  146,  150 
Collenchyma,  64* 
Colocasia,  52* 
Colorado    coniferous    forest,    Plate 

XII 
Coloration   of  autumn   leaves,   121, 

122 

Colors  of  flowers,  424 
Columella,  241,  242* 
Common  ferns,  311-320* 
Common  mosses,  293-301* 
Common  receptacle,  134,  135* 
Comparative  sections  of  fruit,  150* 
Compass  plant,  nearly  vertical  leaves 

of,  100* 

Competition,  447-451* 
Composite,  135*,  400 
Composite  head,  134,  135* 
Compound  cyme,  137* 


544 


INDEX 


Compound  leaves,  91,  92* 

Compound  pistil,  128 

Compound  umbel,  135* 

Couceptacle,  212 

Condensed  stems,  54*,  56 

Cone,  326*,  327 

Confer  vales,  184-192* 

Confervas,  184-192* 

Conidia,  245 

Coniferales,  364-375* 

Coniferous  forest,  Plate  XII 

Conifers,  364-375* 

Conjugates,  193*,  194* 

Continuity  of  protoplasm,  99,  100 

Coral  fungus,  264* 

Cordaitege,  340,  Plate  VIII 

Cordyceps,  253* 

Core,  148 

Cork,  71 

Corn,  aerial  roots  of,  21* 

Corn,  cross  section  of  stem  of,  58* 

Corn,  grain  of,  7*,  505* 

Corn,  section  of  root  tip  of,  24* 

Corn,  selection  among,  504-507* 

Corn  stem,  structure  of,  57,  58* 

Cornus  canadensis,  Frontispiece 

Corolla,  123*,  126* 

Cortex,  65* 

Cortex  of  root,  24* 

Corymb,  133 

Cotton,  526*,  527* 

Cotton,  hybridizing  of,  511,  512 

Cotyledon,  5-14*,  18*,  370*,  374 

Cotyledon,  disposition  made  of,  14 

Cover  (operculum),  299,  300* 

Crossing,  501 

Cross  pollination,  420,  501,  512 

Crowberry,  rolled-up  leaf  of,  466* 

Cryptogams,  354 

Cup  fungi,  250,  251* 

Cup  (volva)  of  gill  fungi,  266,  267 

Cup  (cupule)  of  Marchantia,  282* 

Cuspidate,  App.  I 

Cuticle,  9* 

Cuticle,  unequal  development  of,  by 

epidermis  cells,  117* 
Cutin,  117 
Cutting  leaves,  418* 
Cyanophycese,  174-178* 
Cycadales,  360-363* 
Cycads,  360-363* 
Cycas,  361* 
Cyme,  136,  137* 
Cypress,  111,  Plate  III 
Cystocarp,  216,  217*,  219* 


Dahlia,  thickened  roots  of,  23* 

Daily  movements  of  leaves,  97,  98* 

Damping  off,  247 

Dandelion,  50* 

Darwin,  Charles,  154,  420,  448 

Date  palms,  57*,  518 

Datura,  11,  419,  443* 

Decay,  230,  231 

Deciduous,  121 

Defenses  against  animals,  413-419* 

Definite  annual  growth,  42 

Dehiscing,  146*,  147 

Descent  of  water,  75*,  76 

Desert,  Sahara,  482* 

Deserts  of  United  States,  493-495 

Desmids,  193*,  194 

Determinate  inflorescence,  136* 

Deutzia  leaves,  96*,  97* 

De  Vries,  499,  513 

Diadelphous,  127,  128* 

Diagrams,  floral,  131* 

Diastase,  10 

Diatomales,  195,  196* 

Diatoms,  195,  196* 

Dichogamy,  429* 

Dicksonia,  309,  Plate  VII 

Dicotyledonese,  398-400 

Dicotyledonous  plants,  18,  397-400 

Dicotyledonous  stem,  annual,  gross 
structure  of,  59,  60* 

Dicotyledonous  stem,  cross  section 
of,  60-67* 

Dicotyledonous  stem,  minute  struc- 
ture of,  60-66* 

Dicotyledonous  stem,  rise  of  water 
in,  73-75* 

Dicotyledons,  398-400 

Dimorphous  flowers,  432* 

Dioecious,  124* 

Dionsea,  412* 

Disk  flowers,  134,  135* 

Dispersal  of  seeds,  439-446* 

Dispersal  of  seed  plants,  436-446* 

Distinct,  123*,  126,  127,  129*,  130* 

Diurnal  position,  97,  98* 

Divided  leaves,  90,  91* 

Division,  153 

Dodder,  22*,  23 

Dormant  buds,  87 

Double  fertilization,  383,  384* 

Double  flowers,  139 

Downy  mildew,  244,  247 

Draparnaldia,  190* 

Drip  leaves,  462* 

Drosera,  410*,  411* 


INDEX 


545 


Drought,  endurance  of,  464 
Drought  plants,  459 
Drupe,  148*,  149,  150* 
Dry  fruits,  140-148* 
Duckweed,  462* 
Duct,  25* 
Dulse,  214 

Earth  star,  268,  269* 
Ecological  groups,  459-473* 
Ecology,  plant,  definition  of,  2,   3, 

407 

Economic  botany,  500-535* 
Economic  botany,  definition  of,  3 
Ectocarpus,  206*,  207 
Egg  apparatus,  383* 
Egg  cell,  165,  166,  183*,  189*,  204, 

212 

Egg,  osmosis  in,  37* 
Elater,  285,  287* 
Elder,  pollen  grain  of,  382* 
Elliptical,  App.  I 
Elm,  43* 
Elm  bud,  85* 
Emarginate,  App.  I 
Embryo,  5-12* 
Embryo  sac,    144*,    356,   373,    382, 

383*,  384* 
Endocarp,  150* 
Endosperm,   5-9*,  362*,   372*,  373, 

384,  385*  . 
Energy,  source  of,  in  plants,  7,  110, 

111 

Enzymes,  10,  232 
Epidermis,  24*,  25*,  60*,  61*,  64*, 

103-105* 

Epidermis,  uses  of,  117*,  118 
Epigynous,  130* 
Epigyny,  395 
Epiphytes,  19*,  47*,  309,  310,  471- 

473* 

Equisetales,  325-329* 
Equisetinese,  325-329* 
Equisetum,  325-329* 
Ergot,  253* 

Eubasidiomycetes,  258,  264-269* 
Euglena,  170*,  171 
Euphorbia  splendens,  417* 
Evergreen,  121 
Evolution,  153,  156 
Evolution  of  algse,  225,  226 
Evolution  of  sex,  223,  224 
Excretion  of  water,  116-120 
Existence,  struggle  for,  447-451,  497 
Exocarp,  149,  150* 


Exogenous,  65 

Explosive  fruits,  439 

External  character  of  dicotyledonous 

stem,  59 
External  character  of  monocotyled- 

onous  stem,  57* 
Eyes  of  potato,  52*,  53 

Fall  of  the  leaf,  93*,  120-122 
Family,  153 

Fermentation,  10,  11,  231,  232 
Fern,  life  history  of,  319 
Ferns,  309-324* 
Fertilization,  138-145*,  166 
Fibrous  roots,  23* 
Fibrous-vascular  bundles,   24,    58*, 

59-61*,  65-67*,  103*,  105*,  106* 
Fibro-vascular  bundles  of  ferns,  311, 

312* 

Ficus  elastica,  80,  465* 
Ficus  religiosa,  drip  leaf  of,  462* 
Filament,  127* 
Filicales,  311-320* 
Filicinese,  309-324* 
Fission  plants,  178 
Fittest,  survival  of  the,  498 
Fixation  of  carbon,  107 
Flagellates,  170* 

Flax,  cross  section  of  stem  of,  62* 
Fleshy  fruits,  uses  of,  444,  445,  518- 

520 

Fleshy  roots,  23* 
Floating  seeds,  441,  442 
Floral  diagrams,  131* 
Floral  envelopes,  123* 
Floral  organs,  138-141* 
Flower,  123-145*,  358 
Flower,  bud  scar,  45*,  46* 
Flower  buds,  position  of,  45*,  46*, 

83*,  84* 

Flower,  development  of,  387*,  388 
Flower,  evolution  of,  393-396 
Flower,  nature  of,  138-140* 
Flower,  organs  of,  123* 
Flower,  plan  of,  123*,  130,  131* 
Flower,  symmetry  of,  125*,  395 
Flowers,  ecology  of,  420-435* 
Flytrap,  Venus,  412* 
Follicle,  146*,  147 
Food  cycle,  163,  164 
Food  in  embryo,  9-11* 
Food  products.    See  Plant  products 
Food,  storage  of,  in  root,  23*,  25,  26* 
Food,  storage  of,  in  stem,  52-54*, 

78,  79 


546 


INDEX 


Forest  map,  491 
Forest  region,  490-492 
Formations,  plant,  474-480* 
Formative  tissue,  64*,  65*,  67*,  366*. 

See  growing  point 
Fossil  plants,  339-341,  Plate  VIII 
Foxglove,  leaf  of,  90* 
Free  central  placentation,  129*,  130 
Frond,  308*,  309 
Frost,  action  of,  121 
Fruit,  146-150* 
Fruit  bud,  83*,  84* 
Fucus,  210-212*,  Plate  IV 
Funaria,  294*,  296*,  298*,  299*,  301* 
Fungi,  172,  227-273* 
Fungi,  life  histories  of,  272,  273 
Fungi,  origin  and  evolution  of,  274 
Fusion  of  parts,  395 

Gametangium,  204 

Gamete,  166,  182,  185*,  186,  204 

Gametophyte,  220 

Gametophyte,  degeneration  of,  404- 
406 

Gamopetalous,  126 

Gamosepalous,  126 

Gastromycetes,  268*,  269* 

Geaster,  268,  269* 

Generations,  alternation  of,  220,  278, 
345-350 

Generative  cells  in  pollen  tube,  142*, 
363,  372*,  373,  382* 

Genus,  153 

Geography,  plant,  definition  of,  2 

Geography,  plant,  of  the  United 
States,  489-495 

Geotropism,  31,  32*,  39,  44 

Germ  diseases,  235,  236 

Germination,  6-14* 

Germination,  chemical  changes  dur- 
ing, 6,  7 

Gill  fungi,  265-267* 

Gills,  266,  267* 

Ginkgo,  motile  sperms  of,  363 

Gloeocapsa,  174* 

Glceotrichia,  176*,  177 

Glucose,  108 

Grafting,  68*,  69* 

Grain,  7*,  147,  505*,  514-516 

Grape  sugar,  79,  108 

Grapevine  blight,  247   • 

Grasses,  398,  514,  515 

Grass  pistil,  421* 

Gravity,  31,  39,  44 

Gray,  Asa,  46 


Green  algae,  179-204* 

Green  algae,  reproductive  organs  of, 

204 

Green  algae,  summary  of,  203 
Green  felt,  197,  200* 
Green  layer  of  bark,  71,  72 
Ground  tissue,  313 
Growing  point,  24*,  280*,  316* 
Growth,  measurement  of,  in  stem,  17* 
Growth,  secondary,  65-67* 
Guard  cells,  104* 
Gulf  weed,  212*,  213 
Gymnosperm,  life  history  of,  375 
Gymnospermae,  359-375* 
Gymnosperms,  128,  359-375* 

Hsematococcus,  181 

Hairs,  417,  418 

Hairs  on  leaves,  119 

Hairs,  root,  15,  16*,  24,  25*,  27*,  28 

Hairs,  stinging,  418* 

Halberd-shaped,  App.  I 

Half-inferior  ovary,  130* 

Half  parasites,  408 

Halophytes,  459,  468*,  469,  483* 

Hard  bast,  25*,  60-66* 

Haustoria,  22*,  248 

Head,  134* 

Heart-shaped,  App.  I 

Heart  wood,  71,  72 

Helianthemum,  487* 

Heliotropic  movements',  101* 

Heliotropism,  101* 

Hepaticse,  279-289* 

Herbs,  46 

Heterocysts,  176* 

Heterogamy,  190,  204,  212,  224 

Heterospory,  320,  322,  324,  351-353 

High   mallow,   provisions   for  cross 

pollination  of,  429* 
Hilum,  5* 
Hoinology,  151 
Honeybee,  leg  of,  422*,  423 
Honey  gland,  423 
Honey  locust  thorn,  44* 
Hop,  twining  of,  49* 
Horse-chestnut,  leaf  arrangement  of, 

95* 

Horsetails,  325-329* 
Host,  22*,  23,  407 
Hot  springs,  plants  in,  178 
Hyacinth,  bulb  of,  53* 
Hybrid,  501 

Hybrid  blackberries,  leaves  of,  510* 
Hybrid  plums,  510* 


INDEX 


547 


Hybridizing,  501,  502*,  509-513* 
Hybrids,  production  of,   501,  502*, 

509-513* 
Hydnum,  265* 
Hydrogen,  28,  29 
Hydrophytes,  459-462*,  482,  488 
Hydropterales,  320 
Hymenium,  264,  267* 
Hymenomycetes,  264-267* 
Hypha,  240 

Hypocotyl,  5*,  6*,  12*,  16 
Hypocotyl,  cross  section  of,  65* 
Hypogynous,  130* 
Hypogyny,  395 

Iceland  moss,  256*,  257 
Imperfect  fungi,  254 
Indefinite  annual  growth,  42 
Indehiscent  fruits,  147-149* 
Indeterminate    inflorescence,    132*, 

133 

Indian  corn,  kernel  of,  7* 
Indian  corn,  root  lip  of,  24* 
Indian   corn,   structure  of  stem  of, 

58* 

India-rubber  plant,  leaf  of,  465* 
Indusium,  308*,  313 
Inferior  ovary,  130*,  135*,  150* 
Inflorescence,  132-137* 
Inflorescence,  determinate,  136, 137* 
Inflorescence,  diagrams  of,  136* 
Inflorescence,    indeterminate,    132*, 

133 

Insectivorous  plants,  409-412* 
Insect  pollination,  422-432* 
Insects,  pollen-carrying  apparatus  of, 

422*,  423 
Insects,    sense    of    smell    of,    423, 

424 

Insects,  vision  of,  424 
Insect  traps,  leaves  as,  409-412* 
Insertion  of  floral  organs,  130* 
Integuments,  356,  362*,  370*,   371, 

"381,  383*,  386* 
Intercellular  spaces,  103*,  111* 
Internal  structure  of  dicotyledonous 

stem,  59-62* 
Internal  structure  of  monocotyledo- 

nous  stem,  57,  58* 
Internode,  16,  17* 
Invasion,  451,  452 
Involucre,  134,  135* 
Iris,  rootstock  of,  51* 
Irish  moss,  214*,  216 
Iron,  28,  29 


Irritability  in  plants,  nature  and  oc- 
currence of,  35,  36,  49,  97-99* 
Irritability  in  roots,  39 
Irritability  of  protoplasm,  35,  36 
Irritability  of  tendrils,  48*,  49 
Isoetes,  338*,  339* 
Isogamy,  186,  204,  224 
Ivy,  aerial  roots  of,  20* 

Jungermanniales,  285-287* 

Keel,  125* 

Kelps,  207*,  208*,  209* 
Kidney-shaped,  App.  I 
Knees,  111,  Plate  III 
Knot  and  wart  fungi,  252* 
Knots,  67,  68* 
Krakatoa,  456 

Labiate,  App.  II 
Lachnea,  251* 
Lamina,  88 
Laminaria,  207* 
Lanceolate,  App.  I 
Lateral  buds,  40*,  46*,  82* 
Leaf,  15-18*,  88-122* 
Leaf,  accumulation  of  mineral  mat- 
ter in,  120 

Leaf  arrangement,  94-101* 
Leaf  bases,  52*,  App.  I 
Leaf  blade,  88 

Leaf  buds,  40,  41,  45*,  46*,  83*,  84* 
Leaf,  fall  of,  120-122 
Leaf  forms,  90 
Leaflet,  85,  86* 

Leaf,  member  of  plant  body,  88 
Leaf  mosaics,  95*,  96,  97* 
Leaf  outlines,  App.  I 
Leaf  scars,  52*,  81* 
Leaf  sections,  102,  103*,  465*,  466* 
Leaf  spine,  416*,  417* 
Leafstalk,  88 
Leaf  tendril,  92* 
Leaf  tips,  App.  I 
Leaf  traces,  105*,  106 
Leafy  liverworts,  285-287* 
Leaves  as  insect  traps,  409-412* 
Leaves,  compound,  91-93* 
Leaves,  cutting,  418* 
Leaves,  divided,  90,  91* 
Leaves,  functions  of,  102-122* 
Leaves,  movements  of,  97-101* 
Leaves  of  xerophytes,  465*,  466* 
Leaves,  simple,  88-91* 
Leaves,  structure  of,  102-122* 


548 


INDEX 


Legume,  147,  443* 

Lenticels,  71 

Lepidodendron,  340,  Plate  VIII 

Leucoplasts,  116 

Lianas,  47*,  48 

Lichen,  254-257,  Plate  V 

Lichens,  nature  of,  255 

Life  history,  chromosome  count  in, 

348 

Light,  exposure  to,  39 
Light,  movements  away  from,  39, 101 
Light,  movements  towards,  101 
Lignification,  367 
Lignin,  114 
Ligule,  338* 
Lilac  mildew,  249*,  250 
Lily,  377*,  378*,  380*,  383*,  384*, 

385* 
Lily,  pollen  grains  producing  tubes 

on  stigma,  142* 
Lime,  120 

Linden,  fruit  cluster  of,  439* 
Linden  wood,  structure  of,  66* 
Linear,  App.  I 
Linnaeus,  153 
Liverworts,  275,  279-289* 
Living   matter,    properties  peculiar 

to,  167 

Living  parts  of  the  stem,  71,  72 
Lobe,  126* 
Locules,  129*,  148* 
Locust,  pinnately  compound  leaf  of, 

92* 

Locust,  thorn  stipules  of,  417* 
Lycoperdon,  268* 
Lycopodinese,  329-339* 
Lycopodium,  330-332* 

Macrocystis,  208* 

Magnesium,  28,  29 

Magnolia,  forking  of,  42,  45*,  46* 

Mallows,  pollination  in,  429*,  430* 

Malt,  10,  11 

Maltose,  79 

Mangrove,  468* 

Maple  fruit,  147* 

Marchantia,  280-285* 

Marchantiales,  280-285* 

Marestail,  air  passages  of,  111* 

Marsilia,  321-323* 

Mat  plants,  51 

Mechanics  of  stem,  58*,  59, 60*,  62*, 

312* 

Medullary  ray,  60*,  66,  71,  72,  367 
Megasporangium,  334 


Megaspore,  322 
Megasporophyll,  334,  369 
Melon,  leaf  of,  89* 
Mesembryanthemum,  465* 
Mesophyll,  103*,  105 
Mesophytes,  459,  467,  468 
Mesquite,  root  system  of,  27 
Metabolism,  i  14,  115 
Metabolism,  digestive,  114 
Metachlamydese,  397,  399,  400 
Micropyle,  6*,  142,  144*,  361,  362*, 

381,  383* 

Microsphsera,  249*,  250 
Microsporangium,  334 
Microspore,  322 
Microsporophyll,  334,  369 
Midrib,  89* 
Mildews,  248-250* 
Mildews,  green  and  yellow,  250* 
Mineral  matter  accumulated  in  the 

leaf,  120 
Mistletoe,  408 
Mixed  buds,  83 
Modified  leaves,  80,  81* 
Moisture  favors  root  growth,  39 
Molds,  239-242* 
Monadelphous,  127,  128* 
Monoblepharis,  247 
Monocotyledoneee,  397,  398 
Monocotyledon ous  plants,  18 
Monocotyledonous  stems,  57-59* 
Monocotyledorious  stems,  growth  of, 

in  thickness,  59 
Monocotyledonous    stems,     rise    of 

water  in,  75* 
Monocotyledons,  397,  398 
Monoscious,  124* 
Moon  wort,  324* 
Morchella,  251,  252* 
Morel,  251,  252* 
Morphology,  1,  151 
Mosaics,  leaf,  95*,  96,  97* 
Moss,  life  history  of,  294-297* 
Mosses,  275,  289-301* 
Moths,  427* 
Mougeotia,  194 

Movement  of  water  in  plants,  73-78* 
Movements  of  leaves,  97-101* 
Mucor,  243* 
Mucorales,  239-242* 
Mucronate,  App.  I 
Mulberry,  149* 
Multiple  fruits,  146,  149*,  150 
Musci,  275,  289-301* 
Mushroom,  265,  267* 


INDEX 


549 


Mutations,  497-499 

Mutilated  seedlings,  growth  of,  8* 

Mycelium,  240* 

Mycorrhiza,  269,  270* 

Myrsiphyllum,  56* 

Myxomycetes,  169  n. 

Naked  buds,  80*,  81* 

Natural  selection,  498 

Nebraska  vegetation,  Plate  IX 

Nectar,  423 

Nectar  glands,  423* 

Nectaries,  423 

Negundo,  radial  and  cross  sections 

of  stem  of,  61* 
Nemalion,  216,  217* 
Nereocystis,  209* 
Nest  fungi,  269* 
Netted  veined,  89*,  90* 
Nettle,  stinging  hair  of,  418* 
Nightshade,  leaf  of,  416* 
Nitella,  201 
Nitrification,  233 
Nitrogen,  28 

Nitrogen,  circulation  of,  231*,  233 
Nitrogen,  fixation  of,  234,  235 
Nocturnal  position,  97,  98* 
Node,  16,  17*,  57,  59 
Nomenclature,  153 
Nostoc,  176,  177 
Notched,  App.  I 

Nucellus,  356,  371,  372*,  381*,  386* 
Nuclear  division,  165* 
Nucleole,  164 
Nucleus,  34,  35*,  104* 
Nut,  147*,  148 
Nutrient  substances,  28,  29 
Nutrition  of  plants,  106-111 

Oak  leaves,  arrangement  of,  94* 

Oat,  root  system  of,  26,  27 

Obcordate,  App.  I 

Obovate,  App.  I 

Obtuse,  App.  I 

Odors  of  flowers,  423,  424 

(Edogonium,  188-190* 

Offensive-smelling  plants,  419 

Oil,  9,  11 

One-celled  green  algaB,  179-184* 

Onion  leaf,  section  of,  53* 

Onoclea,  315* 

Oogonium,  188,  189*,  204,  212 

Oospore,  166,  183*,  189*,  204 

Opening,  490,  531* 

Open  veining,  89*,  90* 


Ophioglossum,  324* 

Opposite  branching,  41* 

Orange,  148* 

Orbicular,  App.  I 

Orchid,  aerial  roots  of  an,  19* 

Order,  153 

Origin  of  sex,  187 

Oscillatoria,  175,  176* 

Osmosis,  36-39* 

Osmosis  in  an  egg,  37* 

Osmosis  in  root  hairs,  39 

Outline  of  classification,  155 

Ovary,  128*,  377* 

Ovate,  App.  I 

Overcrowding,  448-450* 

Ovule,  123*,  128,  356 

Ovule  case,  377*,  380* 

Ovule,  structure  of,  144* 

Oxalis  leaf,  development,  86* 

Oxidation,  7 

Oxygen  making,  7,  28,  29,  113,  115 

Pacific  slope,  490,  494,  495 

Paleobotany,  definition  of,  2 

Palisade  cells,  102,  103*,  105*,  107 

Palmate,  89,  91*,  92* 

Palms,  57*,  111*,  Plate  XIII 

Pampas  region,  451 

Panicle,  135*,  136 

Pansy,  leaf-like  stipules  of,  88* 

Papilionaceous  corolla,  125* 

Papillae  on  stigma  of  a  lily,  142* 

Parallel  veining,  89*,  90 

Paraphysis,  251*,  298* 

Parasites,  22*,  23,  172,  227,  407-409* 

Parasitic  roots,  22*,  23 

Parenchyma,  64*,  106* 

Parietal  placenta,  129*,  130 

Parthenogenesis,  244 

Pea  pod,  443* 

Pea  seedling,  mutilated,  8* 

Pea  seedling  on  whirling  disk,  32* 

Peat,  291,  292 

Peat  bogs,  292 

Peat  moss,  290-292* 

Pedicel,  133* 

Peduncle,  133* 

Peltate,  App.  I 

Penicillium,  250* 

Pepo,  149 

Perennial,  26*,  46 

Perianth,  123*,  126*,  129*,  130*  135*, 

App.  II 

Perianth,  differentiation  of,  394 
Pericarp,  149,  150* 


550 


INDEX 


Perigynous,  130* 

Perigyny,  395 

Peronosporales,  244-247* 

Petal,  123,  377* 

Petiole,  88 

Peziza,  251* 

Phseophycese,  205-213* 

Phanerogams,  354 

Phloem,  312* 

Phosphates,  29 

Phosphorus,  28,  29 

Photosynthesis,  107-115,  162,  172 

Phycomycetes,  239-247* 

Phylloxera,  247 

Physcia,  Plate  V 

Physiology,  plant,  definition  of,  2 

Phytophthora,  246* 

Pine  forests,  457*,  490,  529* 

Pine  needle,  364,  365* 

Pine  seedling,  18* 

Pine  stem,  366-369* 

Pinnse,  leaflets  of  a  pinnately  com- 
pound leaf,  92* 

Pinnate,  89,  90*,  91*,  92* 

Pinnules,  leaflets  of  a  pinnately  twice 
compound  leaf,  98* 

Pistil,  123-126*,  128*,  358,  377* 

Pistillate  flower,  124* 

Pitcher  plant,  409* 

Pith,  58-61*,  65*,  67*,  72,  366* 

Placenta,  129*,  130 

Plains  region,  490,  492,  493 

Plankton,  195 

Plant  breeding,  500-513* 

Plant  cell,  159-161* 

Plant  communities,  447 

Plant  ecology,  definition  of,  2,  3 

Plant  evolution  up  to  pteridophytes, 
306 

Plant  formations,  474-480* 

Plant  geography,  481-495* 

Plant  geography,  definition  of,  2,  3 

Plant  geography  of  United  States, 
489-495* 

Plant  fertilizers,  524 

Plant  fibers,  526-529* 

Plant  food  for  domestic  animals,  523, 
524 

Plant  food  for  human  use,  514-522* 

Plant  fuel,  534 

Plant  grains,  514-516* 

Plant  manufactures,  524,  525* 

Plant  medicines,  522,  523 

Plant  physiology,  definition  of,  2 

Plant  products,  514-535* 


Plant  societies,  447 

Plant  successions,  454-458* 

Plant  timber,  529-533* 

Plants,  destruction  of,  by  animals, 

413 
Plants,  groups  of,  in  relation  to  water 

economy,  459 

Plants  of  uneatable  texture,  415 
Plants,  ornamental,  534,  535 
Plasmolysis,  38* 
Plasmopara,  247 
Platycerium,  309*,  310,  472* 
Pleurococcus,  179-180* 
Pleurotus,  Plate  VI 
Plowrightia,  252* 
Plumule,  5*,  6,  7*,  14,  15 
Plurilocular  sporangia,  206*,  207 
Pod,  441-443* 
Poisonous  plants,  419 
Poisonous  roots,  25 
Poisonous  seeds,  11,  419 
Polar  nuclei,  383* 
Pollarded  trees,  87 
Pollen,  127,  141*,  358 
Pollen-carrying  apparatus,  422*,  423 
Pollen  chamber,  362*,  371 
Pollen,  discharge  of,  140 
Pollen   grain,   germination   of,   142, 

143* 
Pollen  grains,  number  of,  per  ovule, 

144,  145 
Pollen,  protection  of,  426,  427,  434*, 

435 

Pollen  sac,  378*,  379,  380 
Pollen  tubes,  142*,  143* 
Pollination,    142*,    144*,    145,    357, 

371,  420-434* 

Pollution  of  water  supply,  171,  178 
Polyadelphous,  127 
Polypetalous,  126 
Polysepalous,  126 
IViysiphonia,  218,  219* 
Poly trich urn,  296* 
Pomer  148 

Pond  scum,  193-195* 
Pond  zonation,  476-478° 
Pore  fungi,  264,  265* 
Porella,  286*,  287* 
Position  of  buds,  82*,  83,  87* 
Postelsia,  209* 
Potassium,  28,  29 
Potato  blight,  or  rot,  246* 
Potato  tuber,  52*,  53,  78,  79 
Prairies,  492,  493 
Prickle,  416*,  417* 


INDEX 


551 


Prickly  leaves,  416*,  417* 

Primary  root,  6*,  12*,  16*,  19 

Primitive  flowers,  393 

Procambium,  65* 

Products,  plant.    See  Plant  products 

Pro-embryo,  144,  145* 

Promycelium,  259*,  261*,  271 

Propagation  by  roots,  436 

Propagation  of  plants,  436-446* 

Prosenchyma,  64 

Protection  of  plants  from  animals, 

413-419* 
Protection  of  pollen,  426,  427,  434*, 

435 

Proteids,  9*,  11*,  25 
Prothallial  cell,  322*,  323,  335*,  368*, 

372*,  373 

Prothallium,  315-317* 
Protobasidiomycetes,  258-264* 
Protococcales,  179,  184* 
Protonema,  294* 

Protoplasm,  34,  35*,  112,  156-168* 
Protoplasm,  characteristics   of,  34- 

36 

Protoplasm,  circulation  of,  202 
Protoplasm,  continuity  of,  99,  214 
Protoplasm,  structure  of,  157 
Protoplast,  34,  35*,  161 
Protosiphon,  200* 
Protozoa,  157 
Pteridophyta,  306-344* 
Pteridophytes,  306-344* 
Pteridophytes,  evolution  of,  343 
Pteridophytes,  origin  of,  342 
Pteridophytes,  summary  of,  343,  344 
PteridospermsB,  392  n. 
Ptomaines,  232,  236 
Public  health,  236,  237 
Puccinia,  260-263* 
Puffball,  268* 
Pulsating  vacuole,  158* 
Pulvinus,  99* 
Pyrenoid,  160*,  162 
Pythium,  247 

Race,  504 

Raceme,  133* 

Radial  symmetry,   123*,  125,  126*, 

138* 

Radiating  stems,  51 
Radishes,  competition  among,  449* 
Rainfall,  481,  488,  489* 
Raspberry,  436*,  445* 
Ray  flowers,  134*,  135* 
Ray,  medullary,  60*,  66,  71,  72 


Receptacle,  123*,  125*,  130* 
Receptacle  of  brown  algae,  210*,  212 
Receptacles    of    Marchantia,    282*, 

284* 

Red  algae,  213-220* 
Red  algae,  life  habits  of,  214,  215 
Red  algae,  summary  of,  219,  220 
Red  clover,  leaf  of,  98* 
Red  snow,  182 
Regions  of  vegetation,  481 
Reindeer  moss,  257* 
Reproduction,  35 
Reproduction    in    flowering  plants, 

138-145*,  436-446* 
Resin  duct,  366* 
Respiration,  107,  110-115 
Resting  buds,  80 
Resting  condition,  112 
Resurrection  moss,  333 
Retuse,  App.  I 
Rhizoids,  280*,  281* 
Rhizopus,  240,  241* 
Rhodophyceae,  213-220* 
Rhubarb  roots,  26* 
Ricciales,  279,  280* 
Ricciocarpus,  280* 
Rigid  tissue,  312,  313.    See  Scleren- 

chyma 

Ring,  annual,  66*,  68* 
Ringent,  App.  II 
Rise  of  water  in  stems,  73-75* 
Rockweeds,  210-212*,  Plate  IV 
Rocky   Mountain  region,  490,  493, 

494 

Root,  6*,  12*,  13,  15-33* 
Root  absorption,  30 
Root  absorption  and  temperature,  30 
Root,  adaptation  to  work,  33 
Root  cap,  24* 
Root  climbers,  20*,  48 
Root,  dicotyledonous,  section  of,  25* 
Root,  fleshy,  23* 

Root  hair,  15,  16*,  24,  25*,  27*,  28 
Root  pressure,  29*,  30 
Root  sections,  24*,  25* 
Root  system,  26,  27 
Root  tubercles,  234* 
Roots,  absorbing  surface  of,  27 
Roots,  adventitious,  19 
Roots,  aerial,  19-21* 
Roots,  brace,  21* 
Roots,  fibrous,  23* 
Roots,  movements  of  young,  30,  31 
Roots,  parasitic,  22*,  23 
Roots,  pine,  lateral  extension  of,  32* 


552 


INDEX 


Roots,  primary,  6*,  12*,  16*,  19 
Roots,  propagation  by,  436 
Roots,  secondary,  6*,  12*,  19,  32* 
Roots,  selective  action  in,  36 
Roots,    storage   of  nourishment  in, 

23*,  25,  26* 

Roots,  structure  of,  24,  25 
Roots,  tertiary,  19 
Roots,  water,  20 
Rootstock,  51*,  52*,  53 
Rosette  plants,  50*,  51 
Rotation  of  protoplasm,  202 
Rots,  246*,  253,  254 
Russian  thistle,  440*,  447,  448 
Russian  thistle,  spread  of,  451,  452 
Rusts,  260-264* 
Rye  grass,  Plate  I 

Saccharomycetes,  238*,  239 

Sac  fungi,  248-257* 

Sac  fungi,  summary  of,  257 

Sagebrush,  480,  493,  494 

Sage,  pollination  in  flowers  of,  430*, 

431* 

Sagittaria,  leaf  of,  461* 
Sago  palm,  78 
Sahara,  482* 
Salicornia,  483*,  493 
Salt  marsh  plants.    See  Halophytes 
Salt  marshes,  458 
Salts,  29 

Salver-shaped,  App.  II 
Salvinia,  320* 

Sap,  descent  of,  74*,  75*,  77 
Sap,  rise  of,  29*,  30,  74*,  75*,  77 
Saprolegnia,  244* 
Saprolegniales,  242-244* 
Saprophytes,  172,  227,  409 
Sapwood,  72 
Sargassum,  212*,  213 
Scale,  16,  18 
Scalloped,  App.  I 
Scaly  buds,  81 
Schizocarp,  147* 
Schizomycetes,  228-237* 
Scion,  68 

Scirpus,  cross  section  of  stem  of,  58* 
Sclerenchyma,  58*,  59 
Scotch  pine  (Pinus  sylvestris),  366*, 

368*,  370* 
Scouring  rush,  325 
Sea  lettuces,  187* 
Seasonal  plants,  459,  467 
Secondary  growth,  65-67* 
Secondary  root,  6*,  12*,  19 


Secondary  roots,  direction  of,  32* 

Sections,  leaf,  102,  103*,  466* 

Sections,  root,  24*,  25* 

Sections,  wood,  59-68* 

Sedge,  rootstock  of,  51* 

Seed,  5-12*,  355,  356 

Seed  coats,  6,  13 

Seed  dispersal,  438-446* 

Seed  habit,  origin  of,  389-393 

Seed  leaf,  5-14*,  18* 

Seedlings,  6*,  8*,  9-18*,  22 

Seedlings,  mutilated  growth  of,  8* 

Seed  plants,  5-14*,  18* 

Seed  plants,  origin  of,  389 

Seeds,  bitter,  11,  419 

Seeds,  poisons  in,  11,  419 

Selaginella,  332-337* 

Selaginella,  life  history  of,  336 

Selaginella,  summary  of,  337 

Selection  by  plant  breeder,  500-509 

Selection,  natural,  498 

Selective  absorption,  36 

Self  pollination,  420 

Sensitive  plants,  98,  99 

Sepal,  123*,  377 

Sequoia,  42,  46,  73,  482,  495 

Sessile  anthers,  127 

Sessile  leaves,  88 

Sessile  stigma,  128 

Sex,  evolution  of,  223,  224 

Sex,  origin  of,  187 

Sexual  characteristics  given  by  heter- 

ospory,  352 

Shade  plants,  470*,  471 
Shame  vine,  98*,  99 
Shepherd's   purse,    development   of 

embryo  and  ovule,  386* 
Shepherd's    purse,    development   of 

flower  of,  387* 
Shoot,  15 

Short-stemmed  plants,  50*,  51 
Shrubs,  45 
Sieve  plate,  63* 
Sieve    tubes,    61-63*,    67,    72,    76, 

105* 

Sigillaria,  340,  Plate  VIII 
Silica,  120 
Simple  leaves,  91 
Simple  pistil,  128 
Simple  umbel  of  cherry,  133* 
Siphonales,  197-201* 
Siphon  algae,  197-201* 
"  Sleep  "  of  plants,  97,  98* 
Slime  molds,  169 
"Smilax,"  56* 


INDEX 


553 


Smoke  tree,  438* 

Smuts,  259* 

Social  plants,  447 

Soft  bast  (sieve  tubes),  61-63* 

Soil,  arid,  zonation,  476-478* 

Solomon's  seal,  parallel-veined  leaf 
of,  89* 

Sorus,  313 

Spatulate,  App.  I 

Species,  153 

Sperm,  166,  183*,  204,  212 

Spermagonia,  262* 

Spermatia,  262* 

Spennatophyta,  354-401* 

Spermatophyte,  354-401* 

Spermatophytes,  400 

Sphserella,  180-182* 

Sphagnales,  290*,  291* 

Sphagnum,  290*,  291* 

Spike,  133,  134* 

Spine,  416*,  417* 

Spiral  vessel,  62* 

Spirogyra,  166* 

Sporangium,  185*,  204,  313,  314* 

Spore,  166 

Spore  case,  313 

Spore  formation  of  ferns,  313 

Spore  fruit,  321*,  322 

Sporidia,  259*,  261*,  271 

Sporophyll,  314,  315* 

Sporophyll,  arrangement  of,  in 
flowers,  394 

Sporophyte,  220,  277,  299,  318 

Sporophyte,  evolution  of,  402-404 

Sporophyte,  origin  of,  349 

Spot  fungi,  253,  254 

Spreading  growth,  43* 

Spring  spores  of  wheat,  261* 

Spruce,  Douglas,  493,  Plate  XII 

Spur,  fruit,  83*,  84* 

Squash  seed,  section  of,  5* 

Stamen,  123-131*,  358,  368*,  369, 
377*,  378*,  379,  380 

Stamen,  parts  of,  127* 

Staminate  flower,  124* 

Standard,  125* 

Starch,  9-11*,  25,  107-110 

Starch  disappears  during  germina- 
tion, 10 

Starch  in  leaves,  107-110 

Starch  making,  rate  of,  109,  110 

Stem,  15-33*,  40-79* 

Stem,  active  portions  of,  71 

Stem,  comparison  of  monocotyledo- 
nous  and  dicotyledonous,  70 


Stem,  definition  of,  40 
Stem,  dicotyledonous,  minute  struc- 
ture of,  59-67* 

Stem,  early  history  of,  64,  65* 
Stem,  functions  of  cells  of,  71,  72 
Stem,  living  parts  of,  71-79* 
Stem,  modifiability  of,  69 
Stem,  monocotyledonous,  57-59* 
Stem,  nature  of,  40 
Stem,  structure  of,  57-70* 
Stem  structure,  early  history  of,  64, 

65* 

Stems,  climbing,  47-50* 
Stems,  condensed,  54* 
Stems,  storage  of  food  in,  78,  79 
Stems,  twining,  49*,  50* 
Stemless  plants,  50*,  52 
Sterigmata,  266 
Stiffening,    mechanics  for,   58*,   59, 

63*,  64* 

Stigma,  128*,  377 
Stimulus  to  protoplasm,  35,  36,  39 
Stinging  hair,  418* 
Stipa,    cross  section  of    rolled   and 

unrolled  leaves  of,  466* 
Stipe,  208 
Stipules,  80,  82* 
Stock,  69 

Stolon,  with  tips  rooting,  436* 
Stoma,  288,  289*,  301* 
Stomata,  71,  103*,  118,  119 
Stomata,  operation  of,  118 
Stone  fruits,  uses  of,  444,  445 
Stone  worts,  201,  202* 
Storage  of  food  in  the  root,  23*,  25, 

26* 

Storage  of  food  in  the  seed,  8-11 
Storage  of  food  in  the  stem,  52-54*, 

78,  79 

Strawberry,  148* 
Strobilus,  327 

Struggle  for  existence,  447-451,  497 
Style,  128* 
Suberin,  117 
Submerged  leaves,  461* 
Successions,  plant,  454-458* 
Sucking  roots,  22* 
Sugar,  10,  11,  25,  79,  107,  112-116, 

522 
Sugar,  formed  during  germination, 

10,  11 
Sugar  cane,  cross  section  of  a  bundle 

from,  75* 
Sulphates,  29 
Sulphur,  28,  29 


554: 


INDEX 


Summer  spores  of  wheat,  262,  263* 
Sundew,  410*,  411* 
Sun  plants,  470*,  471 
Superior  ovary,  130* 
Supernumerary  buds,  82* 
Survival  of  the  fittest,  498 
Suspensor,  144,  145*,  335*,  336,  385, 

386* 

Swamp  zonation,  476-478* 
Swarm  spores,  185*,  186,  204 
Sweet  pea,  flowers  of,  125* 
Symbiosis,  255 

Symmetry,  123*,  125,  126*,  138* 
Sympetake,  397 
Sympetalous,  126 
Sympetaly,  395 
Syncarpy,  395 
Synergids,  383* 
Synsepalous,  126 
Synsepaly,  395 
Systematic  botany,  definition  of,  2 

Taper-pointed,  App.  I 
Tap  root,  23* 
Taxonomy,  definition  of,  2 
Teeth  of  moss  spore  case,  300* 
Teleutospores,  260*,  261 
Temperate  plant   associations,   482, 

483 

Temperature  and  germination,  7 
Temperature  and  leaf  movement,  97, 

98 
Temperature    and     photosynthesis, 

108,  110 
Temperature   and  respiration,   110- 

112 

Temperature  and  root  absorption,  30 
Temperature  and  root  growth,  39 
Temperature  and  transpiration,  120 
Temperature    and   vegetation,  481- 

488 

Tendril,  48*,  49 
Tendril  climbers,  48*,  92* 
Terminal  bud,  40*,  42*,  43*,  45*,  46*, 

52*,  82*,  83*,  84* 
Terminal  flowers,  132* 
Tertiary  root,  19 
Testa,  5*,  6*,  370*,  375 
Tetrad,  219,  277,  280*,  289*,  369 
Tetraspores,  219* 
Thallophyta,  172-274* 
Thallophytes,  172-274* 
Thallophytes,  summary  of,  304,  305 
Thallus,  172 
Thistle,  Russian,  440*,  447,  448 


Thorns,  416*,  417* 

Thorns  as  branches,  44* 

Tickle  grass,  442* 

Timber  line.  485* 

Tissue,  64,  167 

Toadstool,  265-267* 

Tooth  fungi,  265* 

Toxins,  236 

Tracheids,  311,  312*,  366*,  367 

Transition  from  stamens  to  petals, 

139* 

Transpiration,  107,  116 
Transpiration,  amount  of,  119,  120 
Transportation  by  water,  441,  442 
Tree  ferns,  309,  Plate  VII 
Trees,  45 
Trees,  age  of,  46 
Trichogyne,  216,  217* 
Trimorphous  flowers,  432 
Tropseolum,  petiole,  coiling  of,  49* 
Tropical plantassociations,  481,  482*, 

Plate  XIII 

Tropophytes,  459,  482 
Truffles,  253 
Truncate,  App.  I 
Trunk,  42*,  43* 
Tube  nucleus,  372*,  373,  382*  ' 
Tuber,  52*,  53 
Tuber  brumale,  253 
Tubercles  on  roots,  234* 
Tubular  corolla,  App.  II 
Tumbleweeds,  440*,  441*,  442* 
Turgor,  118 
Turnip  seedling,  16* 
Twayblade,  beetle  on  flower  of,  425* 
Twigs,  40*,  46* 
Twiners,  48*,  49*,  50 
Twining,  rate  of,  50 

Ulothrix,  184-186* 

Ulva,  187 

Umbel,  133*,  135* 

Umbellet,  135*,  136 

Underground  stems,  51*,  52*,  53*, 

54,  78,  79 

Uneatable  plants,  50*,  52,  415-419* 
Unilocular  sporangia,  206* 
Union  of  carpels,  128,  129* 
Union  of  stamens,  127*,  128* 
Unipistillary  fruits,  146 
Unisexual  flowers,  124* 
United  States,  plant  geography   of, 

489-495* 

Upright  growth,  42* 
Uredospores,  262*,  263* 


INDEX 


555 


Uridinales,  260-264* 

Uroglena,  170*,  171,  178 

Uses  of  the  components  of  the  stem, 

72 

Usnia,  257* 
Ustilaginales,  259* 
Ustilago,  259* 

Vacuole,  pulsating,  158* 
Variation,  496,  497,  499 
Variety,  500,  503,  504,  506-508, 

512* 

Vaucheria,  197-200* 
Vegetable  physiology,  2 
Vegetation,  alpine,  484-487*,  493 
Vegetation,  arctic,  484* 
Vegetation,  regions  of,  481 
Vegetation,  temperate,  482,  483 
Vegetation,  tropical,  481,  482* 
Vegetative  organs,  15 
Vein,  89*,  10(5* 
Venation,  89*,  106* 
Venus  flytrap,  411,  412* 
Vernation,  85,  86* 
Vertically  placed  leaves,  100*,  101* 
Vessel,  61*,  72 
Volvox,  182-184* 

Walking  fern,  310* 

Water,  absorption  by  roots,  28 

Water,     amount    transpired,     119, 

120 

Water  bloom,  177 
Water,  excretion  of,  115-120 
Water  fern,  320* 
Water  lily,  flower  of,  130*,  139* 
Water  molds,  242-244* 
Water,  movement  of,  28-30*,  73-78*, 

105,  106*,  116-120 
Water  roots,  20 

Water  supply,  pollution  of,  171,  178 
Weapons  of  plants,  416-419* 
Wedge-shaped,  App.  I 


Weeds,  452,  453 

Wheat  grain,  section  of,  9* 

Wheat,  hybridizing  of,  512*,  513 

Wheat  rust,  260-263* 

Wheat,  selection  among,  507-509 

Wheel-shaped,  App.  II 

Whorl,  123* 

Willow,  adventitious  buds  of,  87 

Willow,  arctic,  484* 

Willow,  flowers  of,  124* 

Wind  pollination,  421* 

Winged  fruits  and  seeds,  439-441*, 

442* 

Wings,  125* 
Winter  buds,  80 

Winter  spores  of  wheat,  260*,  261 
Witches'  broom,  264 
Wood  cell,  25*,  61*,  62*,  72,  75* 
Wood,  coniferous,  366*,  367 
Wood  of  linden,  66* 
Wood  parenchyma,  64 
Wood  sections,  59-68* 
Wood,  structure  of,  61*,  63*,  66* 

Xenia,  384  n. 

Xerophytes,  459,  462-466*,  482,  485 

Xerophytic  leaves,  465*,  466* 

Xylaria,  252 

Xylem,  311,  312*,  366*,  367 

Yarrow,  head  of,  135* 
Yeast,  238*,  239 
Yucca,  495 

Zamia,  360*,  362* 
Zonation,  475-478*,  Plate  X 
Zones,  475-478*,  Plate  X 
Zoosporangium,  204 
Zoospores,  185*,  204 
Zygnema,  194* 
Zygomorphic,  125 

Zygospore,  167,  182,  185*,  186,  193*, 
194*,  204,  206* 


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List  price,  90  cents  ;  mailing  price,  $1.00. 

PRACTICAL  PHYSIOLOGY.  A  text-book  for  higher  schools.  Cloth.  448 
pages.  Illustrated.  List  price,  $1.10  ;  mailing  price,  $1.20. 

HOW  TO  TEACH  PHYSIOLOGY.  A  handbook  for  teachers.  Paper.  52  pages. 
Illustrated.  Mailing  price,  10  cents. 


BLAISDELL'S  physiologies  have  maintained  a  remarkable 
popularity  in  the  educational  world.  That  they  are  to-day 
more  widely  used  than  ever  before  is  owing  in  large  part 
to  the  importance  given  by  the  author  to  the  subject  of  the  care 
and  preservation  of  the  health.  The  books  present  in  clear  and 
simple  language  the  latest  and  most  trustworthy  facts  on  physiology 
and  hygiene.  Important  facts  are  illustrated  by  a  series  of  simple 
experiments  which  the  pupil  can  perform  with  little  or  no  outlay 
for  apparatus.  This  feature  is  peculiar  to  the  Blaisdell  books  and 
has  been  found  no  less  valuable  than  original. 

The  author,  recognizing  the  importance  of  physical  exercise  to 
young  people,  has  included  the  most  approved  of  modern  ideas 
on  the  subject  of  physical  culture. 

The  effects  of  alcoholic  drinks,  tobacco,  and  other  narcotics 
upon  the  bodily  life  are  set  clearly  and  forcibly  before  the  pupils' 
minds  as  the  different  topics  are  treated,  not  in  a  mass  at  the  end 
of  the  volume. 


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TEXT-BOOKS  ON   ASTRONOMY 

By  CHARLES  A.   YOUNG 

Professor  Emeritus  of  Astronomy  in  Princeton  University 

LESSONS  IN  ASTRONOMY.  (Revised  Edition.)  Including  Star  Maps. 
420  pages.  Illustrated.  List  price,  $1.25  ;  mailing  price,  $1.40. 

ELEMENTS    OF   ASTRONOMY.     With  a  Uranography.     464  +  42   pages 

and  four  double-paga  star  maps.     List  price,  $1.60  ;  mailing  price,  $1.75. 

URANOGRAPHY.     From  the  "Elements  of  Astronomy."     Flexible  covers.     42  pages 

and  four  double-page  star  maps.     List  price,  30  cents  ;  mailing  price,  35  cents. 

MANUAL  OF  ASTRONOMY.  611  pages.  Illustrated.  List  price,  $2.25  ; 
mailing  price,  $2.45. 

GENERAL  ASTRONOMY.  A  text-book  for  colleges  and  scientific  schools. 
630  pages.  Illustrated  with  250  cuts  and  diagrams  and  supplemented  with 
the  necessary  tables.  List  price,  $2.75  ;  mailing  price,  $3.00. 

A  SERIES  of  text-books  on  astronomy  for  higher  schools, 
academies,  and  colleges,  prepared  by  one  of  the  most  dis- 
tinguished astronomers  of  the  world,  a  popular  lecturer  and 
a  successful  teacher. 

The  "  Lessons  in  Astronomy  "  was  prepared  for  schools  that 
desire  a  brief  course  free  from  mathematics.  The  book  is  fully 
down  to  date,  and  several  beautiful  plates  of  astronomical  objects 
and  instruments  have  been  inserted  in  the  revised  edition. 

The  "  Elements  of  Astronomy "  is  a  text-book  for  advanced 
high  schools,  seminaries,  and  brief  courses  in  colleges  generally. 
Special  attention  has  been  paid  to  making  all  statements  accurate. 

The  "  Manual  of  Astronomy  "  is  a  new  work  prepared  in  response 
to  a  pressing  demand  for  a  class-room  text-book  intermediate  be- 
tween the  author's  "  General  Astronomy  "  and  his  "  Elements  of 
Astronomy."  It  is  largely  made  up  of  material  drawn  from  the 
earlier  books,  but  rearranged,  rewritten  when  necessary,  and  added 
to  in  order  to  suit  it  to  its  purpose  and  to  bring  it  thoroughly 
down  to  date. 

The  eminence  of  Professor  Young  as  an  original  investigator  in 
astronomy,  a  lecturer  and  writer  on  the  subject,  and  an  instructor 
in  college  classes,  led  the  publishers  to  present  the  "  General 
Astronomy  "  with  the  highest  confidence  ;  and  this  confidence  has 
been  fully  justified  by  the  event.  It  is  conceded  to  be  the  best 
astronomical  text-book  of  its  grade. 


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BERGEN'S  FOUNDATIONS  OF  BOTANY 

By  JOSEPH  Y.  BERGEN, 

Instructor  in  Biology  in  the  English  High  School,  Boston,  and 

Author  of  "Elements  of  Botany." 

v 

412  +  257  pages.    Illustrated.    For  introduction,  $1.50. 


A  HANDBOOK  FOR  THE  USE  OF  TEACHERS 

To  Accompany  Bergen's  Foundations  of  Botany. 

Flexible  cloth.    64  pages.    For  introduction,  30  cents. 

ONE  of  the  notable  text-books  of  the  year  is  "  Foundations 
of  Botany  "  by  Mr.  Bergen,  whose  "  Elements  of  Botany  " 
has  come  to  be  the  most  widely  used  recent  text-book  on 
the  subject  in  the  higher  schools  and  academies  of  the 
country. 

The  "Foundations  of  Botany"  is  sufficient  to  prepare 
for  any  college  or  university  which  accepts  botany  as  an 
entrance  requirement.  It  offers  an  extended  and  compre- 
hensive course  for  schools  that  wish  to  devote  an  entire 
year  to  the  subject,  and  provides  the  teacher  who  has  only 
a  minimum  amount  of  time,  with  the  distinct  advantage  of 
a  considerable  option  as  regards  the  kind  and  amount  of 
work  which  he  shall  present  to  his  classes.  It  represents 
the  latest  and  most  advanced  methods  of  botany  teaching, 
combining  a  standard  text  liberally  illustrated  with  a  com- 
plete course  in  laboratory  work  and  a  key  for  the  study  of 
systematic  botany. 

The  treatment  of  structural  and  physiological  botany  is 
unusually  full  and  has  special  reference  to  the  most  recent 
botanical  knowledge. 

The  flora  includes  seven  hundred  species,  and  is  the  only 
recent,  short,  and  thoroughly  simple  and  intelligible  flora 
of  the  central  and  northeastern  states.  The  descriptions 
are  written  in  the  very  simplest  language  consistent  with 
scientific  accuracy. 

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Boston  New  York  Chicago  San  Francisco 

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BERGEN'S 
ELEMENTS    OF    BOTANY 

REVISED  EDITION 

By  JOSEPH  Y.  BERGEN, 

Recently  Instructor  in  Biology  in  the  English  High  School,  Boston 

Including  Key  and  Flora  for  Northern  and  Central  States.  lamo.  Cloth.  283  4-  257 
pages.  Illustrated.  List  price,  $1.30 ;  mailing  price,  $1.45.  Without  Key  and  Flora. 
List  price,  $1.00;  mailing  price,  $1.10. 

Issued  also  in  three  special  editions  with  a  Key  and  Flora  for  each :  Pacific  Coast 
Edition,  Southern  States  Edition,  and  Rocky  Mountain  Edition.  List  price,  $1.30  each; 
mailing  price,  $1.45. 

BERGEN'S  "  Elements  of  Botany,"  Revised  Edition,  is  designed  to 
furnish  a  half-year  course  in  the  subject  for  students  in  secondary 
schools.     It  covers  all  the   ground    which    ordinary  classes  can 
traverse  in  the  time  indicated,  and  presents  only  those  topics  which  are 
essential  to  an  elementary  course  in  the  science. 

It  differs  from  the  earlier  editions  of  the  "  Elements  "  mainly  in  the 
greater  stress  laid  on  the  topics  of  ecology  and  cryptogamic  botany,  in 
the  somewhat  abbreviated  directions  for  histological  work  on  seed  plants, 
and  in  the  greatly  improved  quality  of  the  illustrations.  Minor  changes 
will  be  found  on  almost  every  page. 

THE   BOOK  IS   CHARACTERIZED 

By  the  natural  method  of  presentation,  introducing  the  pupil  first,  as  Professor  Huxley 
recommended,  to  the  comparatively  familiar  forms  and  processes  of  plant  life. 

By  the  sparing  use  of  technical  terms,  employing  these  only  when  they  are  indispensable 
for  the  sake  of  clearness  or  of  brevity. 

By  the  treatment  of  the  structures  and  the  functions  of  plants  consecutively,  not  in 
widely  separated  portions  of  the  book. 

By  the  intimate  combination  of  laboratory  work  with  discussion,  taking  pains,  however, 
not  to  tell  the  pupil,  either  in  words  or  by  means  of  illustrations,  what  he  is  to  see 
before  he  sees  it  for  himself. 

By  the  accuracy  of  the  illustrations  in  detail,  the  half  tones  being  used  only  to  give  gen- 
eral effects,  never  for  -minute  organs  or  strrccttires. 

By  the  fact  that  four  special  keys  and  floras  have  been  prepared  to  accompany  the  text. 
This  allows  the  student  in  any  part  of  the  country  to  obtain  practice  in  the  determi- 
nation of  species  of  phanerogams,  and  to  get  a  practical  idea  of  their  relationships  and 
classification  by  means  of  a  simply  written  and  inexpensive  flora  of  his  own  region. 

BOTANY   NOTEBOOK 

To  accompany  Bergen's  Text-Books  on  Botany,  and  for  general  use  in  Botanical 
Laboratories  or  for  Secondary  Schools.  Square  4to.  Cloth.  144  pages.  List  price, 
45  cents  ;  mailing  price,  60  cents. 

BERGEN'S  Notebook   was  prepared  with   the  particular  view  of 
minimizing  the  amount  of  routine  dictation  for  both  teacher  and 
pupil  without  doing  any  of  the  latter's  thinking  for  him.     Not 
only  will  it  save  time  and  trouble  but  it  will  also  lead  the  pupil  to  per- 
form neat  and  accurate  work. 


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TEXT-BOOKS   ON    CHEMISTRY 

By  R.   P.  WILLIAMS 
Instructor  in  Chemistry  in  the  English  High  School,  Boston 


List     Mailing 
price      price 

ELEMENTS  OF  CHEMISTRY.   Cloth.  412  pages.  Illustrated  #1.10  $1.20 
INTRODUCTION  TO  CHEMICAL  SCIENCE.    Cloth.    216 

pages.     Illustrated 80  .90 

CHEMICAL  EXERCISES.     Paper.     102  sheets 30  .35 

CHEMICAL  EXPERIMENTS.  General  and  Analytical. 

Boards.  212  pages.  Illustrated 50  .60 

LABORATORY  MANUAL  OF  INORGANIC  CHEMISTRY. 
One  hundred  topics  in  general,  qualitative,  and  quantitative 
chemistry.  Boards.  200  pages.  Illustrated ,  .30  .35 

LABORATORY  MANUAL  OF  GENERAL  CHEMISTRY. 

Boards.    200  pages 25          .30 


THE  "Elements  of  Chemistry"  is  very  fully  and  carefully 
illustrated  with  entirely  new  designs  embodying  many 
original  ideas,  and  there  is  a  wealth  of  practical  experi- 
ments. Exercises  and  problems  follow  the  discussion  of  laws  and 
principles. 

The  subject-matter  is  so  divided  that  the  book  can  be  used  by 
advanced  schools,  or  by  elementary  ones  in  which  the  time  allotted 
to  chemistry  is  short. 

"  Chemical  Exercises "  correctly  adapts  to  the  teaching  of 
chemical  theory  the  method  in  language  and  mathematics  now 
in  vogue  in  our  best  schools. 

"  Chemical  Experiments "  is  for  the  use  of  students  in  the 
chemical  laboratory.  It  contains  more  than  one  hundred  sets  of 
the  choicest  illustrative  experiments,  about  half  of  which  belong 
to  general  chemistry,  the  rest  to  metal  and  acid  analysis. 

The  "  Laboratory  Manual  of  Inorganic  Chemistry"  contains  one 
hundred  sets  of  experiments,  including  brief  treatment  of  qualita- 
tive analysis  of  both  metals  and  nonmetals,  and  a  few  quantitative 
experiments. 


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WENTWORTH    AND   HILL'S 

PHYSICS     (Revised  Edition) 

By  G.  A.  WENTWORTH,  formerly  Professor  of  Mathematics  in  Phillips 

Exeter  Academy,  and  G.  A.  HILL,  formerly  Assistant 

Professor  of  Physics  in  Harvard  University 


I2mo.     Half  leather.     480  pages.     Illustrated.     List  price,  $i.  15  ; 
mailing  price,  $1.25 


WENTWORTH  AND  HILL'S  PHYSICS  aims  to  give  a 
rational  explanation  of  the  more  important  physical  phe- 
.  nomena,  and  to  prepare  the  way  for  further  investigation 
and  study  of  physical  science. 

It  gives  so  much  of  the  subject  as  should  form  a  part  of  the 
programme  of  every  secondary  school,  and  no  more.  The  laws 
and  principles  are  stated  as  clearly  as  possible  and  in  such  order 
that  one  naturally  leads  to  the  next.  The  experimental  methods 
by  which  the  laws  were  established  are  given,  but  as  briefly  as  is 
consistent  with  clearness.  The  aim  is  to  form  scientific  habits  of 
thinking,  rather  than  merely  to  impart  knowledge. 

Laboratory  experiments,  requiring  simple  apparatus  only,  are 
described  for  the  purpose  of  verifying  laws  previously  stated,  not 
for  discovering  laws.  Many  numerical  exercises  are  introduced 
into  each  chapter  for  practice  in  applying  the  principles  of  physics 
to  the  common  problems  of  life.  Review  questions  are  given  at 
the  end  of  every  chapter. 

In  the  present  edition  the  work  has  been  thoroughly  revised  in 
order  to  bring  it  into  harmony  with  the  latest  scientific  thought. 
Special  attention  has  been  given  to  electrolysis,  and  the  sections 
dealing  with  this  subject  have  been  rewritten  in  order  to  bring 
them  down  to  date. 

A  new  chapter  on  some  of  the  applications  of  physics  has  been 
added.  It  is  fully  illustrated  and  describes  the  uses  of  compressed 
air,  artificial  refrigeration,  liquid  air,  water-tube  boilers,  steam 
turbines,  gasoline  engines,  electric  furnaces,  wireless  telegraphy, 
and  Rontgen  rays. 


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